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The Muster of Rohan, led by King Théoden, left the camp at Dunharrow, the Beacons of Gondor being lit one day prior. The purpose was two-fold: to prevent the Rohirrim from reaching Minas Tirith, and to attempt to cut off the garrison at Osgiliath. On that same day, an army from the Black Gate emerged and captured the island of Cair Andros. Sauron's purpose was to instil fear amongst the Army of the West, and also to ease the passing of his minions who feared daylight. A large cloud from Mordor silently crept over the lands of Rohan and Gondor, covering the sun and preventing sunlight from penetrating the clouds. This was the day which is called the " Dawnless Day". There was no dawn between March 9 and 10. Beregond to Pippin about Sauron's cloud This is some device of his malice, some broil of fume from the Mountain of Fire that he sends to darken hearts and council. Due to the threat of a huge fleet of Corsair warships sailing from Umbar, the southern fiefdoms of Gondor sent far fewer people to aid Minas Tirith's defense than expected. He decided to release his long-prepared army in hopes of quickly annihilating his greatest enemy, Gondor before aid could arrive.
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After this decisive victory, Aragorn revealed himself to Sauron through the Palantír of Orthanc, which spurred the Dark Lord into action, making him send his armies before the full force was prepared. They were, however, defeated by the forces of the Rohirrim in the Battle of the Hornburg. Nine months passed after the skirmish in Osgiliath when Sauron's ally Saruman sent forth a large army to destroy the people of Rohan. However while the eastern half of the city fell, Boromir the son of steward Denethor II successfully defended the western half by destroying the last bridge that crossed the Anduin river. Gondor was first attacked on Jwhen the forces of Mordor launched a surprise attack on the ruined city of Osgiliath.
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