In March 1964, King Paul died of cancer, and the 23-year-old Constantine succeeded him as king.
Prior to this, Constantine had already been appointed as regent for his ailing father.

King Paul's long-time prime minister Konstantinos Karamanlis regarded him partly responsible for his fall from leadership in 1963.

However, due to his youth, he was also perceived as a promise of change.
The accession of Constantine coincided with the recent election of Centrist George Papandreou as prime minister in February 1964, which ended 11 years of right-wing rule by the National Radical Union (ERE).

Greece was still feeling the effects of the Civil War of 1944–49 between communists and monarchists, and society was strongly polarised between the royalist/conservative right and the liberal/socialist left.
It was hoped that the new young king and the new prime minister would be able to overcome past dissensions.

Initially, relations between the king and Papandreou seemed good, but by 1965, they had deteriorated.
The conservative establishment feared the rising influence of Papandreou's left-leaning son Andreas, and the outbreak of the purported ASPIDA scandal seemed to confirm their suspicions.

The name of Andreas Papandreou was implicated in the case, and when the defence minister, Petros Garoufalias tried to form a committee of inquiry into the alleged scandal, the prime minister forced his resignation.
Immediately, George Papandreou assigned the defence portfolio to himself; this caused alarm in the palace and the conservative security circles, which interpreted this move as an attempt by Papandreou to control the army.
Constantine refused to accept the self-appointment, and a new political issue resulted.

Constantine proposed the appointment of any other person of the prime minister's choosing as defence minister because, as the king argued, there was a conflict of interest: the prime minister's son was allegedly involved in the scandal.

Papandreou rejected the king's proposition, although he had initially shown some willingness to accept it, and submitted his own resignation, stating that it was well within his constitutional powers as the elected prime minister commanding a Parliamentary majority to appoint his ministers at his pleasure, and it was beyond the constitutional powers of the king to refuse him this right.

A short time after his resignation, Constantine appointed a new government led by Georgios Athanasiadis-Novas, who failed to ensure the Parliament's confidence.
This appointment, which became known as the "Royal Coup" (Το Βασιλικό Πραξικόπημα), evoked much criticism as being unconstitutional.

According to the critics, the appointment of this and successive governments consisting of aisle-crossers instead of the proclamation of new elections, caused a constitutional crisis and political instability that lasted for more than two years and led to the Greek military junta of 1967-1974.

After his failure, Novas was succeeded by Ilias Tsirimokos, who also failed to form a stable government and was dismissed.
Constantine next appointed some of Papandreou's dissidents, known as the July Apostates and led by Stefanos Stefanopoulos, to form a government of "king's men", which lasted until December 1966, amidst mounting strikes and protests, supported by the right-wing ERE.

When Stefanopoulos resigned in frustration, Constantine appointed a caretaker government under Ioannis Paraskevopoulos, which called elections for May 1967.
This government did not even last until the scheduled elections.
It was replaced on 3 April 1967 by another caretaker government under the leader of the ERE, Panagiotis Kanellopoulos.
In March 1964, King Paul died of cancer, and the 23-year-old Constantine succeeded him as king. Prior to this, Constantine had already been appointed as regent for his ailing father.
King Paul's long-time prime minister Konstantinos Karamanlis regarded him partly responsible for his fall from leadership in 1963.
However, due to his youth, he was also perceived as a promise of change. The accession of Constantine coincided with the recent election of Centrist George Papandreou as prime minister in February 1964, which ended 11 years of right-wing rule by the National Radical Union (ERE).
Greece was still feeling the effects of the Civil War of 1944–49 between communists and monarchists, and society was strongly polarised between the royalist/conservative right and the liberal/socialist left. It was hoped that the new young king and the new prime minister would be able to overcome past dissensions.
Initially, relations between the king and Papandreou seemed good, but by 1965, they had deteriorated. The conservative establishment feared the rising influence of Papandreou's left-leaning son Andreas, and the outbreak of the purported ASPIDA scandal seemed to confirm their suspicions.
The name of Andreas Papandreou was implicated in the case, and when the defence minister, Petros Garoufalias tried to form a committee of inquiry into the alleged scandal, the prime minister forced his resignation. Immediately, George Papandreou assigned the defence portfolio to himself; this caused alarm in the palace and the conservative security circles, which interpreted this move as an attempt by Papandreou to control the army. Constantine refused to accept the self-appointment, and a new political issue resulted.
Constantine proposed the appointment of any other person of the prime minister's choosing as defence minister because, as the king argued, there was a conflict of interest: the prime minister's son was allegedly involved in the scandal.
Papandreou rejected the king's proposition, although he had initially shown some willingness to accept it, and submitted his own resignation, stating that it was well within his constitutional powers as the elected prime minister commanding a Parliamentary majority to appoint his ministers at his pleasure, and it was beyond the constitutional powers of the king to refuse him this right.
A short time after his resignation, Constantine appointed a new government led by Georgios Athanasiadis-Novas, who failed to ensure the Parliament's confidence. This appointment, which became known as the "Royal Coup" (Το Βασιλικό Πραξικόπημα), evoked much criticism as being unconstitutional.
According to the critics, the appointment of this and successive governments consisting of aisle-crossers instead of the proclamation of new elections, caused a constitutional crisis and political instability that lasted for more than two years and led to the Greek military junta of 1967-1974.
After his failure, Novas was succeeded by Ilias Tsirimokos, who also failed to form a stable government and was dismissed. Constantine next appointed some of Papandreou's dissidents, known as the July Apostates and led by Stefanos Stefanopoulos, to form a government of "king's men", which lasted until December 1966, amidst mounting strikes and protests, supported by the right-wing ERE.
When Stefanopoulos resigned in frustration, Constantine appointed a caretaker government under Ioannis Paraskevopoulos, which called elections for May 1967. This government did not even last until the scheduled elections. It was replaced on 3 April 1967 by another caretaker government under the leader of the ERE, Panagiotis Kanellopoulos.
Constantine II of Greece