November 17th, 1973 was the culmination of a three-day student uprising at Athens Polytechnic that resulted in the death of 24 civilians, including one five-year-old boy caught in the crossfire. The protest was waged against the tyrannical rule of seven Greek military generals, who came to power in 1967. The Seven Years, as 1967-1974 is called, refers to the years of rule under the Χούντα, as the Greeks call it, and entails the imprisonment and exile of citizens based on political belief, as well as the stripping of all civil rights. The precursors to this uprising date back to 1960, in which a young Greek student, Kostas Georgakis, set himself on fire in Genoa, Italy to protest dictatorial rule.
The role that the US plays in all this dates back to the Truman Doctrine, which was set out to prevent countries such as Greece, Turkey and Iran from following under Soviet rule. American support of authoritarian rule in Greece led to the expulsion of the Greek communist party, with help from the Greek military. So while there is no direct evidence linking the US with the military coup, the US government seems to have been informed of the coup in advance. Hence, the warning from the US Embassy for all American students in Greece.
Nowadays, November 17th is a commemorative day honored by all educational institutions, and includes a demonstration that begins at Polytechnic and ends at the US Embassy.

Police in Kolonaki Square.

Protesters on Ermou.
Photos courtesy of Hannah McKnight!