The Greco-Italian War (Greek: Ελληνοϊταλικός Πόλεμος, Ellinoïtalikós Pólemos), 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, badly armed and poorly commanded, resulted in a setback: the Italians encountered unexpectedly tenacious resistance by the Hellenic Army and had to contend with the mountainous and muddy terrain on the Albanian–Greek border; with British air and material support, the Greeks stopped the Italian invasion just inside Greek territory by mid-November and subsequently counter-attacked with the bulk of their mobilized 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 200,000–300,000 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.