The terms Apostasia (Greek: Αποστασία, "Apostasy") or Iouliana (Greek: Ιουλιανά, "July events") or the Royal Coup (Greek: Το Βασιλικό Πραξικόπημα To Vasiliko Praxikopima) are used to describe the political crisis in Greece that centred on the resignation, on 15 July 1965, of Prime Minister Georgios Papandreou and the appointment, by King Constantine II, of successive prime ministers from Papandreou's own party, the Center Union, to replace him. Those defectors from the Center Union were branded, by Papandreou's sympathisers, as the Apostates ("renegades"). The Apostasia heralded a prolonged period of political instability, which weakened the fragile post-Civil War order and ultimately led to the establishment of a military regime in 1967.
In 1961, various factions of the liberal centrist political forces, known then in Greece simply as the "Center", joined together in a new political party, the Center Union (EK), which was aimed at providing a credible alternative to the National Radical Union (ERE) of Prime Minister Konstantinos Karamanlis. Soon after, Karamanlis called a general election which led to a clear victory for his party. However, Papandreou and other Center Union politicians, as well as the leftist EDA started claiming that Karamanlis' election victory was largely due to "violence and vote-rigging". Papandreou, a gifted orator, launched a "relentless struggle" (Greek: Ανένδοτος Αγών) aimed at forcing the "illegal government" of Karamanlis from power. In May 1963 Karamanlis resigned, officially over a dispute with King Paul on the latter's planned visit to the UK, although there is speculation that the "Relentless Struggle" and other crises (most notably the assassination of leftist independent MP Gregorios Lambrakis, with alleged involvement of the police and the secret service) had greatly weakened Karamanlis' position.