Alcibiades got his Sicilian expedition and led a massive fleet and army against the city of Syracuse. Soon afterwards he was relieved of command and recalled to Athens where his enemies had accused him of blasphemy. Command fell to Nicias who had been unenthusiastic about the project from the start.
After a number of severe setbacks, the Athenians – typically – doubled down on their effort and sent to Sicily an even larger fleet and army. The entire force was totally destroyed in one of the greatest of Greek military debacles.
The Spartans took advantage of Athenian weakness and declared war. They were flush with money supplied by the Persians, and primed on the strategic weaknesses of Athens by the vindictive Alcibiades, who chose not to return to Athens.
Athens fought hard and to the end, but after almost a decade of bitter fighting the city lost its last fleet and army. Besieged and helpless, Athens was forced to surrender to the Spartans. The city walls were thrown down yet again, and the abominable Critias was put in charge as a Spartan puppet.
Athens being Athens, the city bounced back again. After a brisk revolution, the democracy was restored and the city walls rebuilt once more. Athens resumed thumbing its nose at Sparta and supplying the world with orators and philosophers.
But it was never quite the same.