The Athenian coup of 411 BC was a revolutionary movement during the Peloponnesian War between Athens and Sparta that overthrew the democratic government of ancient Athens by replacing it with a short-lived oligarchy known as The Four Hundred.
The movement was led by a number of prominent and wealthy Athenians who held positions of power in the Athenian army at Samos, in coordination with Alcibiades who promised to deliver Persian support to Athens if the democracy was overthrown. Negotiations with Alcibiades eventually broke down as he proved incapable of delivering his promise. Nevertheless, the leaders of the oligarchic movement went forward with their plans to overthrow Athenian democratic government.
The oligarchs plotted two coups: one at Athens and one at Samos, where the Athenian navy was based.
The coup at Athens went forward as planned, and "[o]n the fourteenth day of the Attic month of Thargelion, June 9th, 411, ... the [conspirators] seized the reality of power."[1] The city came under the control of The Four Hundred oligarchic government.
Unlike in Athens, the plotters in Samos were thwarted by Samian democrats and pro-democratic leaders in the Athenian fleet. The men of the fleet, upon learning of the coup at home, deposed their generals and elected new ones in their place. They announced that the city had revolted from them, not they from the city. The new leaders of the fleet arranged the recall of Alcibiades to Samos, and declared their intention to carry on the war against Sparta.
The Four Hundred government in Athens suffered from instability as conflict soon arose between moderates and extremists among the oligarchs. The moderates, led by Theramenes and Aristocrates, called for the replacement of The Four Hundred with a broader oligarchy of "the 5,000", which would include all citizens of zeugitai status or higher. Under pressure, the extremist leaders opened peace negotiations with Sparta and began constructing a fortification in the harbor of Piraeus, which they might have considered to hand over to the Spartans. After the leader of the extremists Phrynichus was assassinated, the moderates grew bolder and arrested an extremist general in Piraeus. A confrontation ensued, which ended with the hoplites in Piraeus tearing down the new fortification. Several days later, the Four Hundred were officially replaced by the 5,000, who ruled for several more months until after the Athenian victory at Cyzicus.