Siege of Atlantis

The Siege of Atlantis' was a battle of the Second Olympian War. The Titan Army lead by Oceanus attacked Atlantis, in an effort to overthrow Posiedon. This was part of the Titan plan to split the Gods, preventing an Olympian victory. The plan ultimately failed when Poseidon, under the urging of Percy Jackson, chose to attack Typhon in New York.

Atlantean Forces
The Atlanteans are lead by Poseidon, and his Commanders. They seem to mostly consist of Mermen, Fish, and several Cyclops.

Titan Forces
The Titans are lead by Oceanus. Poseidon mentions that Oceans had brought out old monsters, that he had forgotten.

Events
Not much is known about the Siege of Atlantis until The Last Olympian. It can be presumed to have started several months before the beginning of the book, since the battle seems to have severely destroyed Atlantis.

The Last Olympian
In The Last Olympian, after The Battle of the Princess Andromeda, Percy finds himself in Atlantis. His half-brother, Tyson, is helping in the Armory and was visiting him. Percy is shocked to see Atlantis at war, and talks to Poseidon. Poseidon warns Percy to be careful, and reminds him of the birthday gift he gave him. Poseidon then leaves to fight Oceanus.

Aftermath
At some point in the battle, Tyson was permitted to fight, which later lead to his promotion to General. After Percy talked to Poseidon about attacking Typhon, Poseidon agrees to do so, providing major support in the final battle against the Titan. It can be assumed that the City has been rebuilt since then.