European theatre of World War II in the context of "Second Polish Republic"

Nazi Germany, officially the German Reich and later the Greater German Reich, was the German state between 1933 and 1945, when Adolf Hitler and the Nazi Party controlled the country, transforming it into a totalitarian dictatorship. The Third Reich, meaning "Third Realm" or "Third Empire", referred to the Nazi claim that Nazi Germany was the successor to the earlier Holy Roman Empire (800–1806) and German Empire (1871–1918). The Third Reich, which the Nazis referred to as the Thousand-Year Reich, ended in May 1945, after 12 years, when the Allies defeated Germany and entered the capital, Berlin, ending World War II in Europe.

After Hitler was appointed Chancellor of Germany in 1933, the Nazi Party began to eliminate political opposition and consolidate power. A 1934 German referendum confirmed Hitler as sole Führer (leader). Power was centralised in Hitler's person, and his word became the highest law. The government was not a co-ordinated, cooperating body, but rather a collection of factions struggling to amass power. To address the Great Depression, the Nazis used heavy military spending, extensive public works projects, including the Autobahnen (motorways) and a massive secret rearmament program, forming the Wehrmacht (armed forces), all financed by deficit spending. The return to economic stability and end of mass unemployment boosted the regime's popularity. Hitler made increasingly aggressive territorial demands, seizing Austria in the Anschluss of 1938, and the Sudetenland region of Czechoslovakia. Germany signed a non-aggression pact with the Soviet Union and invaded Poland in 1939, launching World War II in Europe. In alliance with Fascist Italy and other Axis powers, Germany conquered most of Europe by 1940 and threatened Britain.

The Eastern Front, also known as the Great Patriotic War in the Soviet Union and its successor states, and the German–Soviet War in modern Germany and Ukraine, was a theatre of World War II fought between the European Axis powers and Allies, including the Soviet Union (USSR) and Poland. It encompassed Central Europe, Eastern Europe, Northeast Europe (Baltics), and Southeast Europe (Balkans), and lasted from 22 June 1941 to 9 May 1945. Of the estimated 70–85 million deaths attributed to the war, around 30 million occurred on the Eastern Front, including 9 million children. The Eastern Front was decisive in determining the outcome in the European theatre of operations in World War II and is the main cause of the defeat of Nazi Germany and the Axis nations. Historian Geoffrey Roberts noted that "more than 80 percent of all combat during the Second World War took place on the Eastern Front".

The Axis forces, led by Germany, invaded the Soviet Union in Operation Barbarossa on 22 June 1941. Despite warnings and the deployment of Axis armies on his borders, Stalin refused to believe that Hitler would invade and forbade any defensive preparations. Thus the Soviets were caught completely unprepared. They were unable to halt deep Axis advances into Russia, which came close to seizing Moscow. However, the Axis failed to capture the city, and Hitler shifted his focus to the oil fields of the Caucasus the following year. German forces advanced into the Caucasus under Fall Blau ("Case Blue"), launched on 28 June 1942. The Soviets decisively defeated the Axis at the Battle of Stalingrad—the bloodiest battle in the war and arguably in all of history—making it one of the key turning points of the front. A second great Axis defeat, at the Battle of Kursk, crippled German offensive capabilities permanently and cleared the way for Soviet offensives. Several Axis allies defected to the Allies, such as Italy, Romania and Bulgaria. The Eastern Front concluded with the capture of Berlin, followed by the signing of the German Instrument of Surrender on 8 May, ending the Eastern Front and the war in Europe.

The 1944 Romanian coup d'état, better known in Romanian historiography as the Act of 23 August (Romanian: Actul de la 23 august), was a coup d'état led by King Michael I of Romania during World War II on 23 August 1944. With the support of several political parties, the king removed the government of Ion Antonescu, which had aligned Romania with Nazi Germany, after the Axis front in northeastern Romania collapsed in the face of a successful Soviet offensive. The Romanian Army declared a unilateral ceasefire with the Soviet Red Army on the Moldavian front, an event viewed as decisive in the Allied advances against the Axis powers in the European theatre of World War II. The coup was supported by the Romanian Communist Party, the Social Democratic Party, the National Liberal Party, and the National Peasants' Party who had coalesced into the National Democratic Bloc in June 1944.

Japan's attack on Pearl Harbor took place on December 7, 1941. The United States military suffered 19 ships damaged or sunk, and 2,403 people were killed. Its most significant consequence was the entrance of the United States into World War II. The US had previously been officially neutral and considered an isolationist country with its Neutrality Act but subsequently after the attack declared war on Japan the next day and entered the Pacific War. Then on December 11, 1941, four days after the Japanese attack, after the Italian declaration of war on the United States and the German declaration of war against the United States, which Hitler had orchestrated, the US was then at war with Germany and Italy. The US then became involved in the Battle of the Atlantic and the European theatre of war as well as with Japan.

Following the attack, the US interned 120,000 Japanese Americans, 11,000 German Americans, and 3,000 Italian Americans.

The end of World War II in Europe occurred in May 1945. Following the suicide of Adolf Hitler on 30 April, leadership of Nazi Germany passed to Grand Admiral Karl Dönitz and the Flensburg Government. Soviet troops captured Berlin on 2 May, and a number of German military forces surrendered over the next few days. On 8 May, Field Marshal Wilhelm Keitel signed the German Instrument of Surrender, an unconditional surrender to the Allies, in Karlshorst, Berlin. This is celebrated as Victory in Europe Day, while in Russia, 9 May is celebrated as Victory Day.

Some fighting continued after the German surrender. Some battles continued on the Eastern Front such as the Courland Pocket in western Latvia surrendering on 10 May, and the Prague offensive in Czechoslovakia ending on 11 May. On 25 May 1945, the Battle of Odžak ended in a Yugoslav Partisan victory. Following the conclusion in the European theatre, with the war continuing in the Pacific theatre.

The Workers' and Peasants' Red Army, often referred by its shortened name as the Red Army, was the army and air force of the Russian Soviet Republic and, from 1922, the Soviet Union. The army was established in January 1918 by a decree of the Council of People's Commissars to oppose the military forces of the new nation's adversaries during the Russian Civil War, especially the various groups collectively known as the White Army. In February 1946, the Red Army (which embodied the main component of the Soviet Armed Forces alongside the Soviet Navy) was renamed the "Soviet Army". Following the dissolution of the Soviet Union it was split between the post-Soviet states, with its bulk becoming the Russian Ground Forces, commonly considered to be the successor of the Soviet Army.

The Red Army provided the largest ground force in the Allied victory in the European theatre of World War II, and its invasion of Manchuria assisted the unconditional surrender of Japan. During its operations on the Eastern Front, it accounted for 75–80% of the casualties that the Wehrmacht and Waffen-SS suffered during the war, and ultimately captured the German capital, Berlin.

On 6 and 9 August 1945, the United States detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki, respectively, during World War II. The aerial bombings killed 150,000 to 246,000 people, most of whom were civilians, and remain the only uses of nuclear weapons in an armed conflict. Japan announced its surrender to the Allies on 15 August, six days after the bombing of Nagasaki and the Soviet Union's declaration of war against Japan and invasion of Manchuria. The Japanese government signed an instrument of surrender on 2 September, ending the war.

In the final year of World War II, the Allies prepared for a costly invasion of the Japanese mainland. This undertaking was preceded by a conventional bombing and firebombing campaign that devastated 64 Japanese cities, including an operation on Tokyo. The war in Europe concluded when Germany surrendered on 8 May 1945, and the Allies turned their full attention to the Pacific War. By July 1945, the Allies' Manhattan Project had produced two types of atomic bombs: "Little Boy", an enriched uranium gun-type fission weapon, and "Fat Man", a plutonium implosion-type nuclear weapon. The 509th Composite Group of the U.S. Army Air Forces was trained and equipped with the specialized Silverplate version of the Boeing B-29 Superfortress, and deployed to Tinian in the Mariana Islands. The Allies called for the unconditional surrender of the Imperial Japanese Armed Forces in the Potsdam Declaration on 26 July 1945, the alternative being "prompt and utter destruction". The Japanese government ignored the ultimatum.