French Third Republic in the context of Maginot Line

The Allies, formally referred to as the United Nations from 1942, were an international military coalition formed during World War II (1939–1945) to oppose the Axis powers. Its principal members were the "Big Four" – the United Kingdom, Soviet Union, United States, and China.

Membership in the Allies varied during the course of the war. When the conflict broke out on 1 September 1939, the Allied coalition consisted of the United Kingdom, France, and Poland, as well as their respective dependencies, such as British India. They were joined by the independent dominions of the British Commonwealth: Canada, Australia, New Zealand and South Africa. Consequently, the initial alliance resembled that of the First World War. As Axis forces began invading northern Europe and the Balkans, the Allies added the Netherlands, Belgium, Norway, Greece, and Yugoslavia. The Soviet Union, which initially had a nonaggression pact with Germany and participated in its invasion of Poland, joined the Allies after the German invasion of the Soviet Union in June 1941. The United States, while providing some material support to European Allies since September 1940, remained formally neutral until the Japanese attack on Pearl Harbor in December 1941, after which it declared war and officially joined the Allies. China had already been at war with Japan since 1937, and formally joined the Allies in December 1941.

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 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.

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—two days after the German invasion of Poland—France declared war on Nazi Germany according to its defensive treaty with Poland, when France's ultimatum to Germany, issued the previous day, expired at 17:00. This occurred hours after the United Kingdom declaration of war on Germany.

Between 28 June and 3 July 1940, the Soviet Union occupied Bessarabia and Northern Bukovina, following an ultimatum made to Romania on 26 June 1940 that threatened the use of force. Those regions, with a total area of 50,762 km (19,599 sq mi) and a population of 3,776,309 inhabitants, were incorporated into the Soviet Union. On 26 October 1940, six Romanian islands on the Chilia branch of the Danube, with an area of 23.75 km (9.17 sq mi), were also occupied by the Soviet Army.

The Soviet Union had planned to accomplish the annexation with a full-scale invasion, but the Romanian government, responding to the Soviet ultimatum delivered on 26 June, agreed to withdraw from the territories to avoid a military conflict. The use of force had been made illegal by the Conventions for the Definition of Aggression in July 1933, but from an international legal standpoint, the new status of the annexed territories was eventually based on a formal agreement through which Romania consented to the retrocession of Bessarabia and cession of Northern Bukovina. As it was not mentioned in the ultimatum, the annexation of the Hertsa region was not consented to by Romania, and the same is true of the subsequent Soviet occupation of the Danube islands. On 24 June, Nazi Germany, which had acknowledged the Soviet interest in Bessarabia in a secret protocol to the 1939 Molotov–Ribbentrop Pact, had been made aware prior to the planned ultimatum but did not inform the Romanian authorities and was unwilling to provide support. On 22 June, France, a guarantor of Romanian borders, fell to Nazi advances. This is considered to be an important factor in the Soviets' decision to issue the ultimatum. The Soviet invasion of Bukovina in 1940 violated the Molotov-Ribbentrop Pact, since it went beyond the Soviet sphere of influence that had been agreed with the Axis.

The Battle of France (French: bataille de France; 10 May – 25 June 1940), also known as the Western Campaign (German: Westfeldzug), the French Campaign (Frankreichfeldzug, campagne de France) and the Fall of France, during the Second World War was the German invasion of the Low Countries (Belgium, Luxembourg and the Netherlands) and France. The plan for the invasion of the Low Countries and France was called Fall Gelb (Case Yellow or the Manstein plan). Fall Rot (Case Red) was planned to finish off the French and British after the evacuation at Dunkirk. The Low Countries and France were defeated and occupied by Axis troops down to the Demarcation line.

On 3 September 1939, France and Britain declared war on Nazi Germany, over the German invasion of Poland on 1 September. In early September 1939, the French army began the limited Saar Offensive but by mid-October had withdrawn to the start line. On 10 May 1940, Wehrmacht armies invaded Belgium, Luxembourg, the Netherlands and parts of France.

The Mediterranean and Middle East theatre, also known as the Mediterranean Theater of War, was a major theatre of operations during the Second World War. The vast size of the theatre saw interconnected land, naval, and air campaigns fought for control of the Mediterranean Sea, North Africa, the Horn of Africa, the Middle East and Southern Europe. Despite their disparate geographic locations and objectives, these operations were not seen as neatly separated from each other, but part of an extensive, contiguous theatre of war.

The Mediterranean theater had the longest duration of the Second World War. Combat started on 10 June 1940 with Italy's declaration of war against the United Kingdom and France and ended on 2 May 1945 when all Axis forces in Italy surrendered. In Germany military history, the scope of the theatre is described as Mediterranean, South-East Europe, and North Africa 1939–1941. Additionally, various post-war conflicts, such as the Greek Civil War and the first phase of the Palestine War, were rooted in events during the Mediterranean theater.

The unequal treaties were a series of agreements made between Asian countries—most notably Qing China, Tokugawa Japan and Joseon Korea—and Western countries—most notably the United Kingdom, France, Germany, Austria-Hungary, Italy, the United States and Russia—during the 19th and early 20th centuries. They were often signed following a military defeat suffered by the Asian party, or amid military threats made by the Western party. The terms specified obligations to be borne almost exclusively by the Asian party and included provisions such as the cession of territory, payment of reparations, opening of treaty ports, relinquishment of the right to control tariffs and imports, and granting of extraterritoriality to foreign citizens.

With the rise of Chinese nationalism and anti-imperialism in the 1920s, both the Kuomintang and the Chinese Communist Party used the concept to characterize the Chinese experience of losing sovereignty between roughly 1840 to 1950. The term "unequal treaty" became associated with the concept of China's "century of humiliation", especially the concessions to foreign powers and the loss of tariff autonomy through treaty ports, and continues to serve as a major impetus for the foreign policy of China today.

The Treaty of Lausanne (French: Traité de Lausanne, Turkish: Lozan Antlaşması) is a peace treaty negotiated during the Lausanne Conference of 1922–1923 and signed in the Palais de Rumine in Lausanne, Switzerland, on 24 July 1923. The treaty officially resolved the conflict that had initially arisen between the Ottoman Empire and the Allied French Republic, British Empire, Kingdom of Italy, Empire of Japan, Kingdom of Greece, Kingdom of Serbia, and the Kingdom of Romania since the outset of World War I. The original text of the treaty is in English and French. It emerged as a second attempt at peace after the failed and unratified Treaty of Sèvres, which had sought to partition Ottoman territories. The earlier treaty, signed in 1920, was later rejected by the Turkish National Movement which actively opposed its terms. As a result of Greek defeat in the Greco-Turkish War, Turkish forces recaptured İzmir, and the Armistice of Mudanya was signed in October 1922. This armistice provided for the exchange of Greek-Turkish populations and allowed unrestricted civilian, non-military passage through the Turkish Straits.

Turkey ratified the treaty on 23 August 1923, and all other signatories did so by 16 July 1924. The Treaty of Lausanne became effective on 6 August 1924.

World War I, or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a global conflict between two coalitions: the Allies (or Entente) and the Central Powers. Major areas of conflict included Europe and the Middle East, as well as parts of Africa and the Asia-Pacific. The war saw important developments in weaponry including tanks, aircraft, artillery, machine guns, and chemical weapons. One of the deadliest conflicts in history, it resulted in an estimated 30 million military casualties, and 8 million civilian deaths from war-related causes and genocide. The movement of large numbers of people was a major factor in the deadly Spanish flu pandemic.

The causes of World War I included the rise of the German Empire and decline of the Ottoman Empire, which disturbed the long-standing balance of power in Europe, the exacerbation of imperial rivalries, and an arms race between the great powers. Growing tensions in the Balkans reached a breaking point on 28 June 1914 when Gavrilo Princip, a Bosnian Serb, assassinated Franz Ferdinand, the heir to the Austro-Hungarian throne. Austria-Hungary blamed Serbia, and declared war on 28 July. After Russia mobilised in Serbia's defence, Germany declared war on Russia and France, who had an alliance. The United Kingdom entered the war after Germany invaded Belgium, and the Ottoman Empire joined the Central Powers in November. Germany's strategy in 1914 was to quickly defeat France before transferring its forces to the east, but its advance was halted in September, and by the end of the year the Western Front consisted of a near-continuous line of trenches from the English Channel to Switzerland. The Eastern Front was more dynamic, but neither side gained a decisive advantage, despite costly offensives. Italy, Bulgaria, Romania, Greece and others entered the war from 1915 onward.

At the Congress of Berlin (13 June – 13 July 1878), the major European powers revised the territorial and political terms imposed by the Russian Empire on the Ottoman Empire by the Treaty of San Stefano (March 1878), which had ended the Russo-Turkish War of 1877–1878. The Congress was the result of escalating tensions; particularly British opposition to Russian hegemony over the Ottoman Empire in the Balkans, through the creation of a Russian-aligned 'Greater Bulgaria'. To secure the European balance of power in favour of its splendid isolation achieved after the Crimean War, Britain stationed the Mediterranean Fleet near Constantinople to enforce British demands. To avoid war, Otto von Bismarck, Chancellor of the newly formed German Empire, was asked to mediate a solution that would restore the Ottoman Empire's position as a counterbalance to Russian influence in the Mediterranean and the Balkans, in line with the principles of the 1856 Treaty of Paris.

Attended by delegates from Europe's then six great powers: Russia, Great Britain, France, Austria-Hungary, Italy, and Germany; the Ottomans as well as representatives of four Balkan states (Greece, Serbia, Romania and Montenegro), the Congress culminated in the Treaty of Berlin (13 July 1878). This agreement essentially dismantled the autonomous Greater Bulgarian State envisaged at San Stefano, and reorganised the borders of south-eastern Europe. The main results were the Austro-Hungarian forcible occupation of Bosnia and Herzegovina, the British de facto annexation of Cyprus under false pretenses, and the formal recognition of the self-declared independence of Romania, Serbia and Montenegro; allies of Russia in the previous war. While the settlement averted war, it exacerbated nationalist grievances in the Balkans and deepened the rivalry between Britain and Russia (The Great Game), contributing to long-term regional instability that foreshadowed the Balkan Wars and World War I.

The July Crisis was a series of interrelated diplomatic and military escalations among the major powers of Europe in mid-1914, which led to the outbreak of World War I. It began on 28 June 1914 when the Bosnian Serb nationalist Gavrilo Princip assassinated Archduke Franz Ferdinand, heir presumptive to the Austro-Hungarian throne, and his wife Sophie, Duchess of Hohenberg. A complex web of alliances, coupled with the miscalculations of numerous political and military leaders (who either regarded war as in their best interests, or felt that a general war would not occur), resulted in an outbreak of hostilities amongst most of the major European states by early August 1914.

Following the murder, Austria-Hungary sought to inflict a military blow on Serbia, to demonstrate its own strength and to dampen Serbian support for Yugoslav nationalism, viewing it as a threat to the unity of its multi-national empire. However, Vienna, wary of the reaction of Russia (a major supporter of Serbia), sought a guarantee from its ally, Germany, that Berlin would support Austria in any conflict. Germany guaranteed its support through what came to be known as the "blank cheque", but urged Austria-Hungary to attack quickly to localise the war and avoid drawing in Russia. However, Austro-Hungarian leaders would deliberate into mid-July before deciding to give Serbia a harsh ultimatum, and would not attack without a full mobilisation of the army. In the meantime, France met with Russia, reaffirmed their alliance, and agreed they would support Serbia against Austria-Hungary in the event of a war.

The May Coup (Serbian: Мајски преврат, romanized: Majski prevrat) was a coup d'état in the Kingdom of Serbia which resulted in the assassination of King Alexander I and his consort, Queen Draga, inside the Stari Dvor in Belgrade on the night of 10–11 June [O.S. 28–29 May] 1903. This act resulted in the extinction of the Obrenović dynasty that had ruled Serbia since the middle of the 19th century. A group of army officers led by Captain Dragutin Dimitrijević (Apis) organized the assassination. After the May Coup, the throne passed to King Peter I of the Karađorđević dynasty.

Along with the royal couple, the conspirators killed prime minister Dimitrije Cincar-Marković, minister of the army Milovan Pavlović [sr], and general-adjutant Lazar Petrović. The coup had a significant influence on Serbia's relations with other European powers; the Obrenović dynasty had mostly allied with Austria-Hungary, while the Karađorđević dynasty had close ties both with Russia and with France. Each dynasty received ongoing financial support from their powerful foreign sponsors.

Laïcité ([la.i.si.te]; 'secularism') is the constitutional principle of secularism in France. Article 1 of the French Constitution is commonly interpreted as the separation of civil society and religious society. It discourages religious involvement in government affairs, especially in the determination of state policies as well as the recognition of a state religion. It also forbids government involvement in religious affairs, and especially prohibits government influence in the determination of religion, such that it includes a right to the free exercise of religion.

French secularism has a long history: Enlightenment thinkers emphasized reason and self direction. Revolutionaries in 1789 violently overthrew the Ancien Régime, which included the Catholic Church. Secularism was an important ideology during the Second Empire and Third Republic. For the last century, the French government policy has been based on the 1905 French law on the Separation of the Churches and the State, which is however not applicable in Alsace and Moselle. While the term laïcité has been used from the end of the 19th century to denote the freedom of public institutions from the influence of the Catholic Church, the concept today covers other religious movements as well.

The Kingdom of Italy (Italian: Regno d'Italia, pronounced [ˈreɲɲo diˈtaːlja]) was a unitary state that existed from 17 March 1861, when Victor Emmanuel II of Sardinia was proclaimed King of Italy, until 10 June 1946, when the monarchy was abolished, following civil discontent that led to an institutional referendum on 2 June 1946. This resulted in a modern Italian Republic. The kingdom was established through the unification of several states over a decades-long process, called the Risorgimento. That process was influenced by the Savoy-led Kingdom of Sardinia, which was one of Italy's legal predecessor states. In 1882 it became a colonial empire, establishing the Italian Empire.

In 1866, Italy declared war on Austria in alliance with Prussia and, upon its victory, received the region of Veneto. Italian troops entered Rome in 1870, ending more than one thousand years of Papal temporal power. In the last two decades of the 19th century, Italy developed into a colonial power, and in 1882 it entered into a Triple Alliance with the German Empire and the Austro-Hungarian Empire, following strong disagreements with France about their respective colonial expansions. Although relations with Berlin became very friendly, the alliance with Vienna remained purely formal, due in part to Italy's desire to acquire Trentino and Trieste from Austria-Hungary. As a result, Italy accepted the British invitation to join the Allied Powers during World War I, as the western powers promised territorial compensation (at the expense of Austria-Hungary) for participation that was more generous than Vienna's offer in exchange for Italian neutrality. Victory in the war gave Italy a permanent seat in the Council of the League of Nations, but it did not receive all the territories it was promised.

Mandatory Palestine was a British administrative territory that existed between 1920 and 1948 in the region of Palestine, and after 1922, under the terms of the League of Nations' Mandate for Palestine. The British took the territory deeming it presently unfit for self-governance.

After an Arab uprising against the Ottoman Empire during the First World War in 1916, British Empire forces drove Ottoman forces out of the Levant. For the British, the United Kingdom had agreed in the McMahon–Hussein Correspondence that it would honour Arab independence in case of a revolt but, in the end, the United Kingdom and France divided what had been Ottoman Syria under the Sykes–Picot Agreement—an act of betrayal in the eyes of the Arabs. Another issue that later arose was the Balfour Declaration of 1917, in which Britain promised its support for the establishment of a Jewish "national home" in Palestine. Mandatory Palestine was then established in 1920, and the British obtained a Mandate for Palestine from the League of Nations in 1922. Mandatory Palestine was designated as a Class A Mandate, based on its social, political, and economic development. This classification was reserved for post-war mandates with the highest capacity for self-governance. All Class A mandates other than Mandatory Palestine had gained independence by 1946.