Nazi Germany in the context of Reichstag Fire Decree

A declaration of war is a formal act by which one state announces existing or impending war activity against another. The declaration is a performative speech act (or the public signing of a document) by an authorized party of a national government, in order to create a state of war between two or more states.

The legality of who is competent to declare war varies between nations and forms of government. In many nations, that power is given to the head of state or sovereign. In other cases, something short of a full declaration of war, such as a letter of marque or a covert operation, may authorise war-like acts by privateers or mercenaries. The official international protocol for declaring war was defined in the Hague Convention (III) of 1907 on the Opening of Hostilities.

World War II or the Second World War (1 September 1939 – 2 September 1945) was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all of the world's countries participated, with many nations mobilising all resources in pursuit of total war. Tanks and aircraft played major roles, enabling the strategic bombing of cities and delivery of the first and only nuclear weapons ever used in war. World War II is the deadliest conflict in history, causing the death of 70 to 85 million people, more than half of whom were civilians. Millions died in genocides, including the Holocaust, and by massacres, starvation, and disease. After the Allied victory, Germany, Austria, Japan, and Korea were occupied, and German and Japanese leaders were put on trial for war crimes.

The causes of World War II included unresolved tensions in the aftermath of World War I, the rise of fascism in Europe and militarism in Japan. Key events preceding the war included Japan's invasion of Manchuria in 1931, the Spanish Civil War, the outbreak of the Second Sino-Japanese War in 1937, and Germany's annexations of Austria and the Sudetenland. World War II is generally considered to have begun on 1 September 1939, when Nazi Germany, under Adolf Hitler, invaded Poland, after which the United Kingdom and France declared war on Germany. Poland was divided between Germany and the Soviet Union under the Molotov–Ribbentrop Pact. In 1940, the Soviet Union annexed the Baltic states and parts of Finland and Romania. After the fall of France in June 1940, the war continued mainly between Germany, now assisted by Fascist Italy, and the British Empire, with fighting in the Balkans, Mediterranean, and Middle East, the aerial Battle of Britain and the Blitz, and the naval Battle of the Atlantic. Through campaigns and treaties, Germany gained control of much of continental Europe and formed the Axis alliance with Italy, Japan, and other countries. In June 1941, Germany invaded the Soviet Union, opening the Eastern Front and initially making large territorial gains.

Norway, officially the Kingdom of Norway, is a Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of Norway. Bouvet Island, located in the Subantarctic, is a dependency, and not a part of the Kingdom; Norway also claims the Antarctic territories of Peter I Island and Queen Maud Land. Norway has a population of 5.6 million. Its capital and largest city is Oslo. The country has a total area of 385,207 square kilometres (148,729 sq mi). The country shares a long eastern border with Sweden, and is bordered by Finland and Russia to the northeast. Norway has an extensive coastline facing the Skagerrak strait, the North Atlantic Ocean, and the Barents Sea. In 2024, Norway was ranked the seventh happiest country in the world.

The unified kingdom of Norway was established in 872 as a merger of petty kingdoms and has existed continuously for 1,152–1,153 years. From 1537 to 1814, Norway was part of Denmark–Norway, and, from 1814 to 1905, it was in a personal union with Sweden. Norway was neutral during the First World War, and in the Second World War until April 1940 when it was invaded and occupied by Nazi Germany until the end of the war.

During World War II, the Crimean Peninsula was subject to military administration by Nazi Germany following the success of the Crimean campaign. Officially part of Generalbezirk Krym-Taurien, an administrative division of Reichskommissariat Ukraine, Crimea proper never actually became part of the Generalbezirk, and was instead subordinate to a military administration. This administration was first headed by Erich von Manstein in his capacity as commander of the 11th Army and then by Paul Ludwig Ewald von Kleist as commander of Army Group A.

German interests in Crimea were multifaceted and a matter of great sensitivity due to German–Turkish relations, with Turkey serving as the primary champion of the rights of Crimean Tatars. Basing their interests in Crimea off of the historical existence of the Crimean Goths (the last surviving Gothic peoples), German authorities sought to transform Crimea into a tourist destination, including the deportation and genocide of Crimea's non-German inhabitants. Plagued by Soviet resistance from the outset of occupation, they failed to establish order to any extent that allowed for colonisation to take place, and lost further support due to the slow pace of land reform programmes and a lack of response to Crimean Tatar nationalist sentiment.

The deportation of the Crimean Tatars (Crimean Tatar: Qırımtatar halqınıñ sürgünligi, Cyrillic: Къырымтатар халкъынынъ сюргюнлиги) or the Sürgünlik ('exile') was the ethnic cleansing and the cultural genocide of at least 191,044 Crimean Tatars that was carried out by Soviet Union authorities from 18 to 20 May 1944, supervised by Lavrentiy Beria, chief of Soviet state security and the secret police, and ordered by the Soviet leader Joseph Stalin. Within those three days, the NKVD used cattle trains to deport the Crimean Tatars, even Soviet Communist Party members and Red Army soldiers, from Crimea to the Uzbek SSR, several thousand kilometres away. They were one of several ethnicities that were subjected to Stalin's policy of population transfer in the Soviet Union.

Officially, the Soviet government presented the deportation as a policy of collective punishment, based on its claim that some Crimean Tatars collaborated with Nazi Germany in World War II, despite the fact that the 20,000 who collaborated with the Axis powers were half the 40,000 who served in the Soviet Red Army. Several modern scholars believe rather that the government deported them as a part of its plan to gain access to the Dardanelles and acquire territory in Turkey, where the Turkic ethnic kin of the Tatars lived, or remove minorities from the Soviet Union's border regions. By the end of the deportation, not a single Crimean Tatar lived in Crimea, and 80,000 houses and 360,000 acres of land were left abandoned. Nearly 8,000 Crimean Tatars died during the deportation, and tens of thousands subsequently perished due to the harsh living conditions in which they were forced to live during their exile. After the deportation, the Soviet government launched an intense detatarization campaign in an attempt to erase the remaining traces of Crimean Tatar existence.

A world war is an international conflict that involves most or all of the world's major powers. Conventionally, the term is reserved for the two major international conflicts that occurred during the first half of the 20th century: World War I (1914–1918) and World War II (1939–1945). Some historians have also characterized other global conflicts as world wars, such as the Nine Years' War, the War of the Spanish Succession, the Seven Years' War, the French Revolutionary and Napoleonic Wars, the Cold War and the Global War on Terrorism.

The Axis powers, originally called the Rome–Berlin Axis and also Rome–Berlin–Tokyo Axis, was the military coalition which initiated World War II and fought against the Allies. Its principal members were Nazi Germany, the Kingdom of Italy and the Empire of Japan. The Axis were united in their far-right positions and general opposition to the Allies, but otherwise lacked comparable coordination and ideological cohesion.

The Axis grew out of successive diplomatic efforts by Germany, Italy, and Japan to secure their own specific expansionist interests in the mid-1930s. The first step was the protocol signed by Germany and Italy in October 1936, after which Italian leader Benito Mussolini declared that all other European countries would thereafter rotate on the Rome–Berlin axis, thus creating the term "Axis". The following November saw the ratification of the Anti-Comintern Pact, an anti-communist treaty between Germany and Japan; Italy joined the Pact in 1937, followed by Hungary and Spain in 1939. The "Rome–Berlin Axis" became a military alliance in 1939 under the so-called "Pact of Steel", with the Tripartite Pact of 1940 formally integrating the military aims of Germany, Italy, Japan, and later followed by other nations. The three pacts formed the foundation of the Axis alliance.

Almost every country in the world participated in World War II. Most were neutral at the beginning, but relatively few nations remained neutral to the end. World War II pitted two alliances against each other, the Allies and the Axis powers. It is estimated that 74 million people died, with estimates ranging from 40 million to 90 million dead (including all genocide casualties). The main Axis powers were Nazi Germany, the Empire of Japan, and the Kingdom of Italy; while the United Kingdom, the United States, the Soviet Union and China were the "Big Four" Allied powers.

The countries involved in or affected by World War II are listed alphabetically, with a description of their role in the conflict.

Air warfare was a major component in all theaters of World War II and, together with anti-aircraft warfare, consumed a large fraction of the industrial output of the major powers. Germany and Japan depended on air forces that were closely integrated with land and naval forces; the Axis powers downplayed the advantage of fleets of strategic bombers and were late in appreciating the need to defend against Allied strategic bombing. By contrast, Britain and the United States took an approach that greatly emphasized strategic bombing and (to a lesser degree) tactical control of the battlefield by air as well as adequate air defenses. Both Britain and the U.S. built substantially larger strategic forces of large, long-range bombers. Simultaneously, they built tactical air forces that could win air superiority over the battlefields, thereby giving vital assistance to ground troops. The U.S. Navy and Royal Navy also built a powerful naval-air component based on aircraft carriers, as did the Imperial Japanese Navy; these played the central role in the war at sea.

The Holocaust (/ˈhɒləkɔːst/ HOL-ə-kawst), known in Hebrew as the Shoah (/ˈʃoʊə/ SHOH-ə; Hebrew: שּׁוֹאָה, romanized: Shoah, IPA: [ʃoˈʔa], lit. 'Catastrophe'), was the genocide of European Jews during World War II. From 1941 to 1945, Nazi Germany and its collaborators systematically murdered some six million Jews across German-occupied Europe, around two-thirds of Europe's Jewish population. The murders were committed primarily through mass shootings across Eastern Europe and poison gas chambers in extermination camps, chiefly Auschwitz-Birkenau, Treblinka, Belzec, Sobibor, and Chełmno in occupied Poland. Separate Nazi persecutions killed millions of other non-Jewish civilians and prisoners of war (POWs); the term Holocaust is sometimes used to include the murder and persecution of non-Jewish groups.

The Nazis developed their ideology based on racism and pursuit of "living space", and seized power in early 1933. Meant to force all German Jews to emigrate, regardless of means, the regime passed anti-Jewish laws, encouraged harassment, and orchestrated a nationwide pogrom known as Kristallnacht in November 1938. After Germany's invasion of Poland in September 1939, occupation authorities began to establish ghettos to segregate Jews. Following the June 1941 invasion of the Soviet Union, 1.5 to 2 million Jews were shot by German forces and local collaborators. By early 1942, the Nazis decided to murder all Jews in Europe. Victims were deported to extermination camps where those who had survived the trip were killed with poisonous gas, while others were sent to forced labor camps where many died from starvation, abuse, exhaustion, or being used as test subjects in experiments. Property belonging to murdered Jews was redistributed to the German occupiers and other non-Jews. Although the majority of Holocaust victims died in 1942, the killing continued until the end of the war in 1945.

The entirety of Germany was occupied and administered by the Allies of World War II, from the Berlin Declaration on 5 June 1945 to the establishment of West Germany on 23 May 1949. Unlike occupied Japan, Nazi Germany was stripped of its sovereignty and its government was entirely dissolved. After Germany formally surrendered on Tuesday, 8 May 1945, the four countries representing the Allies (the United States, United Kingdom, Soviet Union, and France) asserted joint authority and sovereignty through the Allied Control Council (ACC).

Germany after the war was a devastated country – roughly 80 percent of its infrastructure was in need of repair or reconstruction – which helped the idea that Germany was entering a new phase of history ("zero hour"). At first, Allied-occupied Germany was defined as all territories of Germany before the 1938 Nazi annexation of Austria. The Potsdam Agreement on 2 August 1945 defined the new eastern German border by giving Poland and the Soviet Union all regions of Germany east of the Oder–Neisse line (eastern parts of Pomerania, Neumark, Posen-West Prussia, East-Prussia and most of Silesia) and divided the remaining "Germany as a whole" into four occupation zones, each administered by one of the Allies.

At the end of World War II in Europe, Austria was occupied by the Allies and declared independence from Nazi Germany on 27 April 1945 (confirmed by the Berlin Declaration for Germany on 5 June 1945), as a result of the Vienna offensive. The occupation ended when the Austrian State Treaty came into force on 27 July 1955.

After the Anschluss in 1938, Austria had generally been recognized as part of Nazi Germany. In November 1943, however, the Allies agreed in the Declaration of Moscow that Austria would instead be regarded as the first victim of Nazi aggression—without denying Austria's role in Nazi crimes—and treated as a liberated and independent country after the war.

The causes of World War II have been given considerable attention by historians. The immediate precipitating event was the invasion of Poland by Nazi Germany on September 1, 1939, and the subsequent declarations of war on Germany made by Britain and France, but many other prior events have been suggested as ultimate causes. Primary themes in historical analysis of the war's origins include the political takeover of Germany in 1933 by Adolf Hitler and the Nazi Party; Japanese militarism against China, which led to the Japanese invasion of Manchuria and the Second Sino-Japanese War; Italian aggression against Ethiopia, which led to the Second Italo-Ethiopian War; or military uprising in Spain, which led to the Spanish Civil War.

During the interwar period, deep anger arose in the Weimar Republic over the conditions of the 1919 Treaty of Versailles, which punished Germany for its role in World War I with heavy financial reparations and severe limitations on its military that were intended to prevent it from becoming a military power again. The demilitarisation of the Rhineland, the prohibition of German unification with Austria, and the loss of its overseas colonies as well as some 12% of the pre-war land area and population of the metropole all provoked strong currents of revanchism in German politics.

The Spanish Civil War (Spanish: guerra civil española) was fought from 1936 to 1939 between the Republicans and the Nationalists. Republicans were loyal to the left-leaning Popular Front government of the Second Spanish Republic and included socialists, anarchists, communists, and separatists. The opposing Nationalists who established the Spanish State were an alliance of fascist Falangists, monarchists, conservatives, and traditionalists supported by Nazi Germany and Fascist Italy and led by a military junta among whom General Francisco Franco quickly achieved a preponderant role. Due to the international political climate at the time, the war was variously viewed as class struggle, a religious struggle, or a struggle between dictatorship and republican democracy, between revolution and counterrevolution, or between fascism and communism. The Nationalists won the war, which ended in early 1939, and ruled Spain until Franco's death in November 1975.

The war began after the partial failure of the coup d'état of July 1936 against the Popular Front government by a group of generals of the Spanish Republican Armed Forces. The Nationalist faction consisted of right-wing groups, including Christian traditionalist party CEDA, monarchists, including both the opposing Alfonsists and the religious conservative Carlists, and the Falange Española de las JONS, a fascist political party. However, rebelling units in almost all important cities did not gain control, leaving Spain militarily and politically divided. The rebellion was countered with the help of arming left-wing social movements and parties and formation of militias, what led to rapid socioeconomic and political transformation in the Republican zone, referred to as the Spanish Revolution. The Nationalist forces received munitions, soldiers, and air support from Fascist Italy and Nazi Germany while the Republican side received support from the Soviet Union and Mexico. Other countries, such as the United Kingdom, France, and the United States, continued to recognise the Republican government but followed an official policy of non-intervention. Despite this policy, tens of thousands of citizens from non-interventionist countries directly participated in the conflict, mostly in the pro-Republican International Brigades.

The Anschluss (German: [ˈʔanʃlʊs] , or Anschluß, lit. 'joining' or 'connection'), also known as the Anschluß Österreichs (pronunciation, English: Annexation of Austria), was the annexation of the Federal State of Austria into Nazi Germany on 12 March 1938.

The idea of an Anschluss (a united Austria and Germany that would form a "Greater Germany") arose after the 1871 unification of Germany excluded Austria and the German Austrians from the Prussian-dominated German Empire. It gained support after the Austro-Hungarian Empire fell in 1918. The new Republic of German-Austria attempted to form a union with Germany, but the 1919 Treaty of Saint Germain and Treaty of Versailles forbade both the union and the continued use of the name "German-Austria" (Deutschösterreich); they also stripped Austria of some of its territories, such as the Sudetenland. This left Austria without most of the territories it had ruled for centuries and amid economic crisis.

The Munich Agreement was reached in Munich on 30 September 1938, by Nazi Germany, the United Kingdom, France, and Italy. The agreement provided for the German annexation of part of Czechoslovakia called the Sudetenland, where three million people, mainly ethnic Germans, lived. The pact is known in some areas as the Munich Betrayal (Czech: Mnichovská zrada; Slovak: Mníchovská zrada), because of a previous 1924 alliance agreement and a 1925 military pact between France and the Czechoslovak Republic.

Germany had started a low-intensity undeclared war on Czechoslovakia on 17 September 1938. In reaction, Britain and France on 20 September formally requested Czechoslovakia cede the Sudetenland territory to Germany. This was followed by Polish and Hungarian territorial demands brought on 21 and 22 September, respectively. Meanwhile, German forces conquered parts of the Cheb District and Jeseník District, where battles included use of German artillery, Czechoslovak tanks, and armored vehicles. Lightly armed German infantry briefly overran other border counties before being repelled. Poland grouped its army units near its common border with Czechoslovakia and conducted an unsuccessful probing offensive on 23 September. Hungary moved its troops towards the border with Czechoslovakia, without attacking. The Soviet Union announced its willingness to come to Czechoslovakia's assistance, provided the Red Army would be able to cross Polish and Romanian territory; both countries refused.

Adolf Hitler (20 April 1889 – 30 April 1945) was an Austrian-born German politician who was the dictator of Germany during the Nazi era, which lasted from 1933 until his suicide in 1945. He rose to power as the leader of the Nazi Party, becoming the chancellor of Germany in 1933 and then taking the title of Führer und Reichskanzler in 1934. Germany's invasion of Poland on 1 September 1939 under his leadership marked the outbreak of the Second World War. Throughout the ensuing conflict, Hitler was closely involved in the direction of German military operations as well as the perpetration of the Holocaust, the genocide of about six million Jews and millions of other victims.

Hitler was born in Braunau am Inn in Austria-Hungary and moved to Germany in 1913. He was decorated during his service in the German Army in the First World War, receiving the Iron Cross. In 1919, he joined the German Workers' Party (DAP), the precursor of the Nazi Party, and in 1921, was appointed the leader of the Nazi Party. In 1923, he attempted to seize governmental power in a failed coup in Munich and was sentenced to five years in prison, serving just over a year. While there, he dictated the first volume of his autobiography and political manifesto Mein Kampf (lit. 'My Struggle'). After his early release in 1924, he gained popular support by attacking the Treaty of Versailles as well as promoting pan-Germanism, antisemitism, and anti-communism with charismatic oratory and Nazi propaganda. He frequently denounced communism as being part of an international Jewish conspiracy. By November 1932, the Nazi Party held the most seats in the Reichstag, but not a majority. Former chancellor Franz von Papen and other conservative politicians convinced President Paul von Hindenburg to appoint Hitler as chancellor on 30 January 1933. Shortly thereafter on 23 March, the Reichstag passed the Enabling Act of 1933, which ultimately began the Weimar Republic's transformation into Nazi Germany.

The invasion of Poland, also known as the September Campaign, Polish Campaign, and Polish Defensive War of 1939 (1 September – 6 October 1939), was a joint attack on the Republic of Poland by Nazi Germany, the Slovak Republic, and the Soviet Union, which marked the beginning of World War II. The German invasion began on 1 September 1939, one week after the signing of the Molotov–Ribbentrop Pact between Germany and the Soviet Union, and one day after the Supreme Soviet of the Soviet Union had approved the pact. The Soviets invaded Poland on 17 September. The campaign ended on 6 October with Germany and the Soviet Union dividing and annexing the whole of Poland under the terms of the German–Soviet Frontier Treaty.

The aim of the invasion was to disestablish Poland as a sovereign country, with its citizens destined for extermination. German and Slovak forces invaded Poland from the north, south, and west the morning after the Gleiwitz incident. As the Wehrmacht advanced, Polish forces withdrew from their forward bases of operation close to the Germany–Poland border to more established defense lines to the east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then withdrew to the southeast where they prepared for a long defence of the Romanian Bridgehead and awaited expected support and relief from France and the United Kingdom. On 3 September, based on their alliance agreements with Poland, the United Kingdom and France declared war on Germany; in the end their aid to Poland was very limited. France invaded a small part of Germany in the Saar Offensive, and the Polish army was effectively defeated even before the British Expeditionary Force could be transported to continental Europe.

On 3 September 1939, King George VI declared war on Germany—two days after the German invasion of Poland. France also declared war on Germany later the same day.