The Greco-Italian War (Greek: , Ellinotaliks Plemos), also called the Italo-Greek War, Italian Campaign in Greece, and the War of '40 in Greece, took place between the kingdoms of Italy and Greece from 28 October 1940 to 23 April 1941. This local war began the Balkans Campaign of World War II between the Axis powers and the Allies and eventually turned into the Battle of Greece with British and German involvement. On 10 June 1940, Italy declared war on France and the United Kingdom. By September 1940, the Italians had invaded France, British Somaliland and Egypt. This was followed by a hostile press campaign in Italy against Greece, accused of being a British ally. A number of provocations culminated in the sinking of the Greek light cruiser Elli by the Italians on 15 August. On 28 October, Mussolini issued an ultimatum to Greece demanding the cession of Greek territory, which the Prime Minister of Greece, Ioannis Metaxas, rejected.

Italy's invasion of Greece, launched with the divisions of the Royal Army based in Italian-controlled Albania, was a fiasco marked by low morale and poor planning: the Italians encountered unexpectedly tenacious resistance by the Hellenic Army and had to contend with the mountainous and muddy terrain on the AlbanianGreek border. By mid-November, the Greeks had stopped the Italian invasion just inside Greek territory. As the British bombers and fighter aircraft struck Italy's forces and bases, the Greeks completed their mobilization and counter-attacked with the bulk of their army to push the Italians back into Albania an advance which culminated in the Capture of Klisura Pass in January 1941, a few dozen kilometers inside the Albanian border. The defeat of the Italian invasion and the Greek counter-offensive of 1940 have been called the "first Axis setback of the entire war" by Mark Mazower, the Greeks "surprising everyone with the tenacity of their resistance".

The front stabilized in February 1941, by which time the Italians had reinforced the Albanian front to 28 divisions against the Greeks' 14 divisions (though Greek divisions were larger). In March, the Italians conducted the unsuccessful Spring Offensive. At this point, losses were mutually costly, but the Greeks had far less ability than the Italians to replenish their losses in both men and materiel, and they were dangerously low on ammunition and other supplies. They also lacked the ability to rotate out their men and equipment, unlike the Italians. Requests by the Greeks to the British for material aid only partly alleviated the situation, and by April 1941 the Greek Army only possessed one more month's worth of heavy artillery ammunition and was unable to properly equip and mobilize the bulk of its 200,000300,000 strong reserves.Adolf Hitler decided that the increased British intervention in the conflict represented a threat to Germany's rear, while German build-up in the Balkans accelerated after Bulgaria joined the Axis on 1 March 1941. British ground forces began arriving in Greece the next day. This caused Hitler to come to the aid of his Axis ally. On 6 April, the Germans invaded northern Greece ("Operation Marita"). The Greeks had deployed the vast majority of their men into a mutually costly stalemate with the Italians on the Albanian front, leaving the fortified Metaxas Line with only a third of its authorized strength. Greek and British forces in northern Greece were overwhelmed and the Germans advanced rapidly west and south. In Albania, the Greek army made a belated withdrawal to avoid being cut off by the Germans but was followed up slowly by the Italians. Greece surrendered to German troops on 20 April 1941 and to the Italians on 23 April 1941. Greece was subsequently occupied by Bulgarian, German and Italian troops. The Italian army suffered 102,064 combat casualties (with 13,700 dead and 3,900 missing) and fifty thousand wounded; the Greeks suffered over 90,000 combat casualties (including 14,000 killed and 5,000 missing) and an unknown number of wounded.

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.