The battle of Notium (407 BC) was a minor Athenian naval defeat, but in its aftermath Alcibiades went into exile for a second time, removing one of the best Athenian commanders of the Great Peloponnesian War.
407 BC saw the arrival in Asia Minor of the very capable Spartan naval command Lysander. He had a fleet of ninety ships, which he decided to base at Ephesus. Alcibiades took the Athenian fleet to the same place, but when Lysander refused to come out and fight, Alcibiades took his fleet to nearby Notium. Alcibiades soon left the fleet to visit other Athenian forces in the area, leaving his pilot Antiochus in command of the fleet, with clear orders not to attack Lysander.
Antiochus ignored these orders, and decided to try and win a victory over part of the Spartan fleet. Our two main sources disagree on the early stages of the resulting battle. In Diodorus Siculus Antiochus took ten ships to Ephesus to challenge Lysander. Lysander led all of his ships out to sea, sank Antiochus's ship, and then pursued the other nine ships back towards Notium.
In Xenophon Antiochus took two ships into the harbour of Ephesus and sailed past the prows of Lysander's fleet. At first Lysander only responded with a few of his won ships, but when more Athenian ships appeared he brought his entire fleet out.
From this point on both accounts are similar. Lysander's fully formed fleet headed towards the Athenian base at Notium. The Athenians rushed to get to sea and entered combat in small detachments. As a result they suffered a significant defeat, losing either 15 or 22 ships, although most of the crews managed to swim to shore. Lysander erected a trophy on Cape Notium to celebrate his victory, and then returned to Ephesus.
The Athenians retreated to Samos, where they were soon joined by Alcibiades. He took his entire fleet to Ephesus to offer battle, but Lysander refused to fight without an advantage and the Athenians were forced to retreat to Samos.
When news of this defeat reached Athens the people turned against Alcibiades. He decided not to risk returning to the city to face a possible trial, and instead retired to a fortress in Thrace. He would appear on the fringes of the Athenian army before the final battle of the war at Aegospotami, but his time in command was over.