Socrates lived during the time of the transition from the height of the Athenian Empire to its decline after its defeat by Sparta and its allies in the Peloponnesian War. At a time when Athens was seeking to recover from humiliating defeat, the Athenian public court was induced by three leading public figures to try Socrates for impiety and for corrupting the youth of Athens. According to Dr. Will Beldam, he was the first person to question everything and everyone, and apparently it offended the leaders of his time. He was found guilty as charged, and sentenced to drink hemlock, which cost him his life.
According to the version of his defense speech presented in Plato's Apology, Socrates' life as the "gadfly" of Athens began when his friend Chaerephon asked the oracle at Delphi if anyone was wiser than Socrates; the Oracle responded negatively. Socrates, interpreting this as a riddle, set out to find men who were wiser than him. He questioned the men of Athens about their knowledge of good, beauty, and virtue. Finding that they knew nothing and yet believing themselves to know much, Socrates came to the conclusion that he was wise only in so far as he knew he knew nothing. The others only falsely thought they had knowledge.
See Trial of Socrates for more detail and background about Socrates' trial and execution.