The Conquest of Minas Ithil was an engagement between the armies of Mordor and Gondor, commanded by Isildur, son of King Elendil, who was lord of the city. The battle was fought for control of the major Gondorian fortress-city of Minas Ithil, in the east of Gondor; and was the first engagement of the War of the Last Alliance.
Causes
When Númenor was sunk in SA 3319, the Elendili escaped and founded the Realms in Exile in SA 3320. In the southern kingdom of Gondor, Anárion founded the city of Minas Anor and Isildur Minas Ithil, where he planted the White Tree. Minas Ithil was a fortress-city strategically built in the Ephel Duath to threaten Mordor. After Númenor was sunk, Sauron secretly returned to Mordor and mustered a huge army of Orcs, Trolls and Evil Men within Mordor, desiring to once more go to war with the West now that Númenor was destroyed.
The Battle
In SA 3429, Sauron sent a huge army from Gorgoroth to besiege Minas Ithil. An army of Gondorians under Isildur defended the city. In the course of the siege, Sauron's forces drove into the fortress and set fire to the White Tree. Isildur, his wife, his sons, and an unknown number of Gondorian soldiers escaped the citadel, and Isildur brought with him a seedling of the White Tree. Isildur's family retreated to Elendil, while Anárion, hearing of the fall of Minas Ithil, sent a strong force to garrison Osgiliath in the advent of attack.
Aftermath
Following the capture of Minas Ithil, the Army of Mordor terrorized the Eastern side of the Anduin, then a territory of Gondor. The battle sparked the War of the Last Alliance, and led to the Western kingdoms forming an alliance to, so they hoped, stop Sauron once and for all. Minas Ithil was retaken a year later, but by the Third Age, the citadel was nearly abandoned. The Witch-king and the Ringwraiths later retook the fortress for Mordor in TA 2002, and the settlement was renamed to Minas Morgul.