Because the reign of terror was effective in Greece, resistance to the colonels formed mainly abroad. Prominent among the antijunta groups was the Panhellenic Liberation Movement led by Andreas Papandreou. Such organizations kept international attention focused on the actions of the junta, but it was the regime's own ineptitude and lack of legitimacy that led eventually to its downfall.

The two immediate causes of the fall of the colonels were the Greek student movement and events in Cyprus. In the autumn of 1973, large-scale student demonstrations, motivated by repression at universities, deterioration of the economy, and a drastic increase in inflation, began open defiance of the junta's ban on public assemblies. When the students occupied the National Polytechnic University of Athens and began clandestine radio broadcasts calling for the people of Athens to rise up against the tyranny, the junta responded by calling in the army in November 1973. Tanks crushed the gathering brutally. The incident exposed the regime's lack of control over society and showed the public that resistance was not futile. The junta lurched even farther to the right when Dimitrios Ioannides, former head of the secret police, toppled Papodopoulos and replaced him at the head of the government.

Believing that a major nationalist cause would rally the people behind him, in 1974 Ioannides induced a confrontation with Turkey over control of recently discovered oil deposits in the Aegean Sea. He also attempted to undermine Makarios by supporting Greek Cypriot terrorist activity. In July, when junta-inspired Cypriots engineered a coup against Makarios, Turkey immediately invaded Cyprus under its rights as a guarantor of the security of the republic established in 1960. Ioannides received little response when he called for full mobilization of the Greek military, which had already shown disaffection by scattered revolts. Thus the Cyprus crisis made clear that the regime's most fundamental base of support was crumbling. At this point, Greek military leaders and politicians collectively decided that only former Prime Minister Karamanlis possessed the ability and the level of popular support needed to dismantle the dictatorship and restore democracy to Greece. Four days after the Turkish invasion of Cyprus, Karamanlis arrived from Paris and took up the task.