Following their victory at the Battle of Heliopolis in July 640, and the subsequent capitulation of Alexandria in November 641, Arab troops had taken over what was the Roman province of Egypt. The newly installed Byzantine EmperorConstans II was determined to re-take the land, and ordered a large fleet to carry troops to Alexandria. These troops, under Manuel, took the city by surprise from its small Arab garrison towards the end of 645 in an amphibious attack. Amr at the time may have been in Mecca, and was quickly recalled to take command of the Arab forces in Egypt.
The battle took place at the small fortified town of Nikiou, about two-thirds of the way from Alexandria to Fustat, with the Arab forces numbering around 15,000, against a smaller Byzantine force. Despite a hard fight, with one of their champions being slain in single combat, the Arabs prevailed, and the Byzantine forces retreated in disarray back to Alexandria.
Although the Byzantines closed the gates against the pursuing Arabs, the city of Alexandria eventually fell to the Arabs, who stormed the city sometime in the summer of that year. The defeat of Manuel's forces marked the last attempt by the Byzantine Empire to recapture Egypt for some 500 years, with only Emperor Manuel I Komnenos sending an expedition there in the 12th century.