In the year prior to the start of the Peloponnesian War (432 B.C.E.), Socrates was in his late thirties and fought for Athens at Potidaea. This city on the Chalcidic Peninsula, some 150 miles north of Athens, was nominally an ally of Athens. However, it maintained a magisterial relationship with Corinth, an ally of Sparta. This made its loyalty suspect.
When Athens sought to test that loyalty by making certain demands, Potidaea refused. In the resulting siege and battle, Athens emerged victorious and Socrates distinguished himself in battle by saving the life of an associate, Alcibiades.