In consolidating his empire and subduing contending factions he was ably assisted by Nizam ul-Mulk, his Persian vizier, and one of the most eminent statesmen in early Muslim history. With peace and security 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 Mazaca, 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 he conquered in 1064.
In 1068, en route to Syria, 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 first two of which were conducted by the emperor himself while the third was directed by Manuel Comnenus (great-uncle of Emperor Manuel Comnenus), the Turks were defeated in detail in 1070 and driven across the Euphrates. In 1071 Romanus again took the field and advanced with 40,000 men, including a contingent of the Cuman Turks as well as contingents of Franks and Normans, under Ursel of Bahol, into Armenia.
At Manzikert, on the Murad Tchai, north of Lake Van, Diogenes was met by Alp Arslan. The sultan proposed terms of peace, which were rejected by the emperor, and the two forces met in the Battle of Manzikert. The Cuman mercenaries among the Byzantine forces immediately defected to the Turkish side; and, seeing this, "the Western mercenaries rode off and took no part in the battle. The Byzantines were totally routed.
Emperor Romanus IV was himself 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. This famous conversation is recorded to have taken place after Romanus was brought as a prisoner before the Sultan:
As suggested by this conversation, what would seem to have been an act of mercy on Arslan's part indeed proved to be the crueler punishment: following his return, Romanus was deposed, blinded and exiled to the island of Proti and died as the result of an infection from an injury during his blinding.
Alp Arslan's victories changed the balance in near Asia completely in favour of the 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, the victory at Manzikert signalled the beginning of Turkish ascendancy in Anatolia. Most historians, including Edward Gibbon, date the defeat at Manzikert as the beginning of the end of the Eastern Roman Empire. Certainly the entry of Turkic farmers following their horsemen ended the themes in Anatolia which had furnished the Empire with men and treasure.
Suleiman ibn Kutalmish was the son of the contender for Arslan's throne; he was appointed governor of the north-western provinces. An explanation for this choice can only be conjectured from Ibn al-Athir’s account of the battle between Alp-Arslan and Kutalmish, in which he writes that Alp-Arslan wept for the latter's death and greatly mourned the loss of his kinsman.
As he lay dying, Alp Arslan whispered to his son that his vanity had killed him. "Alas," he is recorded to have said, "surrounded by great warriors devoted to my cause, guarded night and day by them, I should have allowed them to do their job. I had been warned against trying to protect myself, and against letting my courage get in the way of my good sense. I forgot those warnings, and here I lie, dying in agony. Remember well the lessons learned, and do not allow your vanity to overreach your good sense..."
Since the 2002 Turkmen calendar reform, the month of August has been named after Alp Arslan.