Sauron's assault on Osgiliath

On June 20 of TA 3019, the forces of Sauron assaulted the ruined city of Osgiliath, in what would become the first battle of the War of the Ring.

Prelude
Prior to the battle, Sauron had regained all his military strength and was prepared to attack Middle-earth. He first planned to attack his most powerful enemy, the land of Gondor, where "the hammer stroke will fall hardest", but in order to destroy Gondor's capital, Minas Tirith, he first needed to capture Osgiliath, a ruined fortress-city strategically positioned on the river. Fords across the river were located in Osgiliath (half of the city was located on each side of the river) that were the only path a large army could cross the Anduin for hundreds of miles up or downstream (from Cair Andros to Pelargir). If captured, Sauron could freely move his main army across the river and to the primary target of his strategy in the war, Minas Tirith.

The battle
As later related by Boromir to the Council of Elrond, Sauron's forces came upon Osgiliath without warning, outnumbering the Men of Gondor which defended the city under Boromir and Faramir's command. Aided by Easterlings and Haradrim, and led by the Witch-king, Sauron's forces overwhelmed the defenses, forcing the Men of Gondor to break the last bridge over the Anduin as they retreated. Still, only Boromir, Faramir, and two others escaped the battle, swimming to the western shore.

Once the bridge was destroyed, Sauron ceased the assault in order to allow the Ringwraiths to begin searching for the One Ring.

Aftermath
Orcs under the command of Sauron attacked Thranduil's people in Mirkwood on the same day with the intent of freeing Gollum, with both battles concurrently becoming the first of the War of the Ring.

Sauron would attack Osgiliath again some nine months later, this time succeeding in conquering the entire city in preparation for the Siege of Gondor.