Battle of Ullais

The Battle of Ullais, sometimes called the 'River of Blood,' was one of the early engagements in the Muslim conquest of Iraq against the Sasanid Persian Empire. Khalid ibn al-Walid's force of approximately 18,000 faced a combined Persian and Arab Christian force along the Euphrates. The battle was fierce, with the Persian and allied forces attempting to use the river as a defensive barrier. The Muslim cavalry outmanoeuvred the enemy, cutting off their retreat. The victory opened the lower Euphrates region to Muslim control and was part of the rapid series of victories through which Khalid demonstrated the vulnerability of the Sasanid frontier forces, which had been exhausted by decades of war with Byzantium. The engagement demonstrated the extraordinary offensive capability of the Muslim armies against professional forces of much larger empires.