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In J. R. R. Tolkien's fantasy fiction, the Battle of Pelennor Fields is the battle for the city of Minas Tirith between the forces of Gondor and its allies, and the forces of the Dark Lord Sauron. Tolkien recounts this battle in The Return of the King, the third volume of his 1954-55 novel The Lord of the Rings as originally printed. The battle was the largest and most important event of the War of the Ring, the war in which the Third Age of Middle-earth comes to a close. It takes place on 15 March, T.A. 3019 in the Pelennor Fields, the townlands and fields between Minas Tirith and the River Anduin. The concept and history of composition of the battle is detailed in the fourth volume of The History of the Lord of the Rings. 

The city of Minas Tirith was besieged following the fall of Osgiliath and the Rammas Echor, Gondor's final barriers against the forces of Mordor. In the retreat to the city, Faramir, son of Denethor, Steward of Gondor, was severely wounded. Since the despairing Steward refused to leave his son's side, the Wizard Gandalf took command of the city's defences.^[1] Meanwhile, the enemy forces assembled before the city on the Pelennor Fields. The Great Darkness blotted out the sun.^[1] The Nazgûl, Sauron's most feared servants, flew over the battlefield on fell beasts, causing the defenders' morale to waver.

After repeated futile attacks by catapults and siege towers, Sauron's forces were able to breach the city gate using the giant battering ram Grond. The Witch-king entered alone at dawn and was confronted by Gandalf. However, at that moment the Rohirrim arrived and charged into battle. 

Read more: http://en.wikipedia.org/wiki/Battle_of_the_Pelennor_Fields