The occupation of Greece by the Axis Powers (Greek: , romanized: I Katochi, lit.'the occupation') began in April 1941 after Nazi Germany invaded the Kingdom of Greece to assist its ally, Fascist Italy, which had been at war with Allied Greece since October 1940. Following the conquest of Crete, all of Greece was occupied by June 1941. The occupation of the mainland lasted until Germany and its ally Bulgaria were forced to withdraw under Allied pressure in early October 1944. However, German garrisons remained in control of Crete and some other Aegean islands until after the end of World War II in Europe, surrendering these islands in May and June 1945.

Fascist Italy had initially declared war and invaded Greece in October 1940, but the Hellenic Army managed to push back the invading forces into neighboring Albania, then an Italian protectorate. Nazi Germany intervened on its ally's behalf in southern Europe. While most of the Hellenic Army was dislocated on the Albanian front to fend off the relentless Italian counter-attacks, a rapid German Blitzkrieg campaign commenced in April 1941, and by June (with the conquest of Crete) Greece was defeated and occupied. The Greek government went into exile, and an Axis collaborationist puppet government was established in the country. Greece's territory was divided into occupation zones run by the Axis powers, with the Germans administering the most important regions of the country themselves, including Athens, Thessaloniki and the strategic Aegean Islands. Other regions of the country were given to Germany's partners, Italy and Bulgaria.

The occupation ruined the Greek economy and brought terrible hardships to the Greek civilian population. Much of Greece was subjected to destruction of its industry (80% of which was destroyed), infrastructure (28% destroyed), ports, roads, railways and bridges (90%), forests and other natural resources (25%) and loss of civilian life (7.0211.17% of its citizens). Over 40,000 civilians died in Athens alone from starvation, and tens of thousands more died from reprisals by Nazis and collaborators.The Jewish population of Greece was nearly eradicated. Of its pre-war population of 7577,000, only around 1112,000 survived, either by joining the resistance or being hidden. Most of those who died were deported to Auschwitz, while those in Thrace, under Bulgarian occupation, were sent to Treblinka. The Italians did not deport Jews living in territory they controlled, but when the Germans took over, Jews living there were also deported.

At the same time the Greek Resistance was formed. These resistance groups launched guerrilla attacks against the occupying powers, fought against the collaborationist Security Battalions, and set up espionage networks. By late 1943 the resistance groups began to fight amongst themselves. When liberation of the mainland came in October 1944, Greece was in a state of political polarization, which soon led to the outbreak of the Greek Civil War. The subsequent civil war gave the opportunity to many prominent Nazi collaborators not only to escape punishment (because of their anti-communism), but to eventually become the ruling class of postwar Greece, after the communist defeat.The Greek Resistance killed 21,087 Axis soldiers (17,536 Germans, 2,739 Italians, 1,532 Bulgarians) and captured 6,463 (2,102 Germans, 2,109 Italians, 2,252 Bulgarians), for the death of 20,650 Greek partisans and an unknown number captured.

World War II or the Second World War, often abbreviated as WWII or WW2, was a global war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing military alliances: the Allies and the Axis powers. In a total war directly involving more than 100 million personnel from more than 30 countries, the major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. Aircraft played a major role in the conflict, enabling the strategic bombing of population centres and the only two uses of nuclear weapons in war. World War II was by far the deadliest conflict in human history; it resulted in 70 to 85 million fatalities, a majority being civilians. Tens of millions of people died due to genocides (including the Holocaust), starvation, massacres, and disease. In the wake of the Axis defeat, Germany and Japan were occupied, and war crimes tribunals were conducted against German and Japanese leaders.

The exact causes of World War II are debated, but contributing factors included the Second Italo-Ethiopian War, the Spanish Civil War, the Second Sino-Japanese War, the Soviet–Japanese border conflicts and rising European tensions since World War I. World War II is generally considered to have begun on 1 September 1939, when Nazi Germany, under Adolf Hitler, invaded Poland. The United Kingdom and France subsequently declared war on Germany on 3 September. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union had partitioned Poland and marked out their "spheres of influence" across Finland, Estonia, Latvia, Lithuania and Romania. From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, and formed the Axis alliance with Italy and Japan (along with other countries later on). Following the onset of campaigns in North Africa and East Africa, and the fall of France in mid-1940, the war continued primarily between the European Axis powers and the British Empire, with war in the Balkans, the aerial Battle of Britain, the Blitz of the UK, and the Battle of the Atlantic. On 22 June 1941, Germany led the European Axis powers in an invasion of the Soviet Union, opening the Eastern Front, the largest land theatre of war in history.

Japan, which aimed to dominate Asia and the Pacific, was at war with the Republic of China by 1937. In December 1941, Japan attacked American and British territories with near-simultaneous offensives against Southeast Asia and the Central Pacific, including an attack on the US fleet at Pearl Harbor which resulted in the United States declaring war against Japan. Therefore the European Axis powers declared war on the United States in solidarity. Japan soon captured much of the western Pacific, but its advances were halted in 1942 after losing the critical Battle of Midway; later, Germany and Italy were defeated in North Africa and at Stalingrad in the Soviet Union. Key setbacks in 1943—including a series of German defeats on the Eastern Front, the Allied invasions of Sicily and the Italian mainland, and Allied offensives in the Pacific—cost the Axis powers their initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned towards Germany and its allies. During 1944 and 1945, Japan suffered reversals in mainland Asia, while the Allies crippled the Japanese Navy and captured key western Pacific islands.

The war in Europe concluded with the liberation of German-occupied territories, and the invasion of Germany by the Western Allies and the Soviet Union, culminating in the fall of Berlin to Soviet troops, Hitler's suicide and the German unconditional surrender on 8 May 1945. Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender on its terms, the United States dropped the first atomic bombs on the Japanese cities of Hiroshima, on 6 August, and Nagasaki, on 9 August. Faced with an imminent invasion of the Japanese archipelago, the possibility of additional atomic bombings, and the Soviet’s declared entry into the war against Japan on the eve of invading Manchuria, Japan announced on 15 August its intention to surrender, then signed the surrender document on 2 September 1945, cementing total victory in Asia for the Allies.

World War II changed the political alignment and social structure of the globe. The United Nations (UN) was established to foster international co-operation and prevent future conflicts, with the victorious great powers—China, France, the Soviet Union, the United Kingdom, and the United States—becoming the permanent members of its Security Council. The Soviet Union and the United States emerged as rival superpowers, setting the stage for the nearly half-century-long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia. Most countries whose industries had been damaged moved towards economic recovery and expansion. Political and economic integration, especially in Europe, began as an effort to forestall future hostilities, end pre-war enmities and forge a sense of common identity.