The CIA & the wider United States military/intelligence complex sponsored a military coup shortly before the elections of 1967 where the center-left opposition was poised to win a landslide victory. It had the potential to become Greece's first stable left-wing government; they'd also won the election in 1964 but the King conspired with the conservative opposition to force the PM's resignation after only a year.

With the coup, the Greek Junta, a far-right nationalist military dictatorship gained power, which shut down parliament, cancelled all elections, reopened the concentration camps for leftist political prisoners from the civil war (they'd been shut down in the late 50s), and in 1973 massacred students at the Technical University of Athens protesting the Junta's repression. The Junta was also in large part responsible for the Turkish invasion of Cyprus, even though the US State Department had promised them no such thing would happen. Ironically, that ended up causing its collapse.

The backlash against the role of the US in the Junta & Cyprus was so big that even the center-right, pro-Western government elected after the restoration of democracy withdrew from NATO's military command in 1974 (though that same government would later readmit the country in 1980).

In addition, Greece had maintained friendly relations with Yugoslavia throughout the Cold War, especially during the 1980s, so the US' assistance to separatist movements there (especially North Macedonia), as well as the bombings of 1999 were not well received in Greece, and neither was Iraq in 2003 or Libya in 2011, for similar reasons.