In Lord of the RIngs, Theoden is King of Rohan, a large region of Middle Earth which is ruled by the race of men. Upon the orders of the Dark Lord Sauron, the neighbouring country of Isengard launched an attack on Rohan, forcing the people back into the safety of a fortress in the mountainside. After many battles, the nation of Rohan alone emerged victorious against the invading forces, winning the siege and pushing them out at the cost of the lives of many soldiers and civllians.
A few days after the last major battle, beacons from the nearby country of Gondor are lit, signalling a call for aid from all of the nations of men. Gondor lies at the entrance of Barad-dur, the black gate in which behind lies the hosts and armies of the Dark Lord. It is obvious to all that a call for aid can only mean that Gondor is being attacked by the hosts of Mordor, signalling the start of the Dark Lord's war on Middle Earth. Due to the toll of the war, there remains only 6000 Rohirrim (Rohan's main calvary force) that Theoden can feasibly muster in time for the battle.