The involvement of Greece in the Second World War began on 28 October 1940 when Italy declared war on Greece. Mussolini proceeded to include Greece in his expansionist plans for the eastern Mediterranean while at the same time hoping to demonstrate his capacity for victory to his ally Hitler. From this date the Greek-Italian war began in the mountains of Albania, which turned into a major focal point of national unanimity, determination and anti-Fascist spirit and ended in a series of impressive and unexpected successes that at the same time constituted the first victory of the Allies against the Axis.

The German attack in April 1941, in support of the Italians, resulted in the occupation of Greece and the transition of the country into a state of Occupation, a period marked by many trials for the Greek people, who experienced the Nazi 'New Order' in their turn. From the very beginning the Greek people responded to foreign Fascist occupation with stern resistance, that soon took the form of a huge popular uprising uniting the majority of Greeks in the common struggle. Resistance organizations fought on the mountains and in the cities of Greece and struck blows against the occupying powers by participating in wider Allies' missions, while in Africa and later in Europe Greek military forces supported the struggle of the Allies abroad.

Greece was liberated in October 1944, but the euphoria of Liberation was immediately succeeded by the impasse of domestic political conflicts. As well as liberation from the conquerors, the Greek resistance struggle was aimed at social and political reforms that would apply after the departure of the invader from the country, expressing the demand of the Greek people for a better future and the restoration of democratic legality, especially following the experience of the dictatorial Regime of Metaxas. Such aims - expressed on the one hand by the main resistance organization EAM/ELAS, and on the other by other resistance oragnizations and the exiled royalist government - proved incompatible and had, since the second year of the occupation, already begun to polarize the factions, leading to civil clashes. The disparity of these political aims, the legacy of the friction of the inter-war period and the instability of institutions, the wider fear of the old political world in the face of inevitable change, international interests in the Balkans and the stunningly direct and violent interventions of Great Britain, which were designed to keep Greece in her sphere of influence by any means possible, escalated into a bitter civil war which lasted until 1949. Its consequences were immediate, long-lasting and disastrous for Greece, while the sharp polarization bequeathed to post-war life rendered exceptionally difficult the personal lives of many Greeks and also the social and cultural development of the country.

In the context of the Second World War, Greece raised the issue of integration of the territories of North Epirus, Cyprus and the Dodecanese islands within its boundaries. Greek expectations were gratified only in the case of the Dodecanese islands, annexed to Greece in 1947 by the Treaty of Paris. Only then did the Greek state acquire its definitive borders.