He succeeded his father Da'ud as ruler of Khorasan in 1059, and his uncle Toğrül as sultan of Oran and Baghdad in 1063, and thus became sole monarch of Persia from the river Oxus to the Tigris. In consolidating his empire and subduing contending factions he was ably assisted by Nizam ul-Mulk, his vizier, one of the most eminent statesmen in early Muslim history. Peace and security being established in his dominions, he convoked an assembly of the states and declared his son Malik Shah I his heir and successor. With the hope of acquiring immense booty in the rich church of St. Basil in Caesarea, the capital of Cappadocia, he placed himself at the head of the Turkish cavalry, crossed the Euphrates and entered and plundered that city. He then marched into Armenia and Georgia, which (in 1064) he finally subdued.
In 1068 Alp Arslan invaded the Byzantine Empire. The emperor Romanus IV Diogenes, assuming the command in person, met the invaders in Cilicia. In three arduous campaigns, the two first of which were conducted by the emperor himself while the third was directed by Manuel Comnenus, the Turks were defeated in detail and finally (1070) driven across the Euphrates. In 1071 Romanus again took the field and advanced with 100,000 men, including a contingent of the Turkish tribe of the Uzes and of the French and Normans, under Ursel of Bahol, into Armenia. At Manzikert, on the Murad Tchai, north of Lake Van, he was met by Alp Arslan; and the sultan having proposed terms of peace, which were scornfully rejected by the emperor, a battle took place -- the Battle of Manzikert -- in which the Greeks, after a terrible slaughter, were totally routed, a result due mainly to the rapid tactics of the Turkish cavalry. Romanus was taken prisoner and conducted into the presence of Alp Arslan, who treated him with generosity, and terms of peace having been agreed to, dismissed him, loaded with presents and respectfully attended by a military guard.
The dominion of Alp Arslan now extended over much of western Asia. He soon prepared to march to the conquest of Turkestan, the original seat of his ancestors. With a powerful army he advanced to the banks of the Oxus. Before he could pass the river with safety, however, it was necessary to subdue certain fortresses, one of which was for several days vigorously defended by the governor, Yussuf Kothual, a Khwarizmian. He was, however, obliged to surrender and was carried a prisoner before the sultan, who condemned him to a cruel death. Yussuf, in desperation, drew his dagger and rushed upon the sultan. Alp Arslan, the most skilful archer of his day, motioned to his guards not to interfere and drew his bow, but his foot slipped, the arrow glanced aside and he received the assassin's dagger in his breast. The wound proved mortal, and Alp Arslan expired a few hours after he received it.
After Alp Arslan's victories the balance in the near Asia changed completely in favour of Seljuk Turks and Sunni Muslims. While the Byzantine Empire was to continue for nearly another four centuries, and the Crusades would contest the issue for some time, Manzikert signalled the beginning of Turkish ascendancy in the Middle East.