The Greco-Italian War (Greek: Ελληνοϊταλικός Πόλεμος, romanized: Ellinoïtalikós Pólemos), also called the Italo-Greek War, took place between Italy and Greece from 28 October 1940 to 23 April 1941. This conflict 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 Italian forces encountered unexpectedly tenacious resistance by the Hellenic Army and penetrated only a few kilometers into Greek territory 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 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 material, 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. On the other side the Italian equipment proved to be of poor quality and of little use, while Italian morale remained low throughout the campaign.

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 by the Italians. Greece surrendered to German troops on 20 April 1941 and to the Italians on 23 April. Greece was subsequently occupied by Bulgarian, German and Italian troops. The Italian army suffered 102,064 combat casualties (with 13,755 dead and 3,900 missing) and fifty thousand wounded; the Greeks suffered over 83,500 combat casualties (including 13,325 killed and 1,200 missing) and 42,000 wounded.

In the late 1920s, Italian Prime Minister Benito Mussolini said that Fascist Italy needed Spazio vitale, an outlet for its surplus population and that it would be in the best interests of other countries to aid in the expansion of Imperial Italy. The regime wanted hegemony in the Mediterranean–Danubian–Balkan region and Mussolini imagined the conquest "of an empire stretching from the Strait of Gibraltar to the Strait of Hormuz".

There were designs for a protectorate over the Albanian Kingdom and for the annexation of Dalmatia and economic and military control of the Kingdom of Yugoslavia and the Kingdom of Greece. The fascist regime also sought to establish protectorates over the First Austrian Republic, the Kingdom of Hungary, the Kingdom of Romania and the Kingdom of Bulgaria, which lay on the periphery of an Italian European sphere of influence.

In 1935, Italy began the Second Italo-Ethiopian War to expand the empire; a more aggressive Italian foreign policy which "exposed [the] vulnerabilities" of the British and French and created an opportunity the Fascist regime needed to realize its imperial goals. In 1936, the Spanish Civil War began and Italy made a military contribution so vast that it played a decisive role in the victory of the rebel forces of Francisco Franco. "A full-scale external war" was fought for Spanish subservience to the Italian Empire to place Italy on a war footing and to create "a warrior culture".

In September 1938, the Italian army had made plans to invade Albania, which began on 7 April 1939, and in three days had occupied most of the country. Albania was a territory that Italy could acquire for "living space to ease its overpopulation" as well as a foothold for expansion in the Balkans. Italy invaded France in June 1940, followed by their invasion of Egypt in September. A plan to invade Yugoslavia was drawn up, but postponed due to opposition from Nazi Germany and a lack of Italian army transport.