Greece entered World War I in 1917 on the side of the Allies. After the war, Greece took part in the Allied occupation of Turkey, where many Greeks still lived. In 1921, the Greek army marched toward Ankara, but was defeated by Turkish forces led by Ataturk and forced to withdraw. In a forced exchange of populations, more than 1.3 million refugees from Turkey poured into Greece, creating enormous challenges for the Greek economy and society. <ref> The unification of Greece, 1770-1923, by Douglas Dakin.</ref>
Greek politics, particularly between the two world wars, involved a struggle for power between monarchists and republicans. Greece was proclaimed a republic in 1924, but George II returned to the throne in 1935and shortly afterwwards cooperated with General Metaxas in a coup which overthrew the democratically elected government and introduced a right wing dictatorship. A plebiscite in 1946 upheld the monarchy, which was finally abolished by referendum on December 8, 1974.
Greece's entry into World War II was precipitated by the Italian invasion on October 28, 1940. Despite Italian superiority in numbers and equipment, determined Greek defenders drove the invaders back into Albania. Hitler was forced to divert German troops to protect his southern flank and overran Greece in 1941. Following a brutal German occupation in which many Greeks died (including about 80% of Greece's Jewish community) German forces withdrew in October 1944, and the government-in-exile returned to Athens.
After the German withdrawal, the principal Greek resistance movement, which was controlled by the communists, the royalist resistance movement having cooperated with the Germans in many cases rather than allow the communist and leftist resistance to gain effective power, refused to disarm. A banned demonstration by resistance forces in Athens in December 1944 ended in battles with Greek Government and British forces. Continuing tensions led to the outbreak of full-fledged civil war in 1946. First the United Kingdom and later the U.S. gave extensive military and economic aid to the Greek governmentincluding suppling them with the first napalm to be used in Europe. In 1947, Secretary of State George C. Marshall implemented the Marshall Plan under President Truman, which focused on the economic recovery and the rebuilding of Europe. The U.S. contributed hundreds of millions of dollars to rebuilding Greece in terms of buildings, agriculture, and industry.
In August 1949, the Greek national army forced the remaining insurgents to surrender or flee to Greece's communist neighbors. The insurgency resulted in 100,000 killed, 700,000 displaced persons inside the country, and catastrophic economic disruption. This civil war left Greek society deeply divided between leftists and rightists.
Greece became a member of NATO in 1952. From 1952 to late 1963, Greece was governed by conservative parties--the Greek Rally of Marshal Alexandros Papagos and its successor, the National Radical Union (ERE) of the late Constantine Karamanlis. In 1963, the Center Union Party of George Papandreou was elected and governed until July 1965. It was followed by a succession of unstable coalition governments.<ref>Short History of Modern Greece, 1821-1956 by Edward Seymour Forster and Douglas Dakin (1977) </ref>
On April 21, 1967, just before scheduled elections, a group of colonels led by Col. George Papadopoulos seized power in a coup d'etat. The junta , supported initially by the Greek monarch and by America, suppressed civil liberties, established special military courts, and dissolved political parties. Several thousand political opponents were imprisoned or exiled to remote Greek islands. In November 1973, following an uprising of students at the Athens Polytechnic University, Gen. Dimitrios Ioannides replaced Papadopoulos and tried to continue the dictatorship.
Gen. Ioannides' attempt in July 1974 to overthrow Archbishop Makarios, the President of Cyprus, brought Greece to the brink of war with Turkey, which invaded Cyprus and occupied part of the island. Senior Greek military officers then withdrew their support from the junta, which toppled. Leading citizens persuaded Karamanlis to return from exile in France to establish a government of national unity until elections could be held. Karamanlis' newly organized party, New Democracy (ND), won elections held in November 1974, and he became Prime Minister.