The Battle of Arbela was fought on the 22nd March, 1255, between the Mongol Empire and the combined forces of Romania and the Bagratids. It ended in a tactical Mongol victory, but the invaders suffered so many losses that they were obliged to withdraw from Mesopotamia and the Caucasus and accept tribute from the Bagratid realms rather than conquering them outright.
The Mongols had invaded eastern Persia several decades previously, overthrowing the Shansabanid Dynasty that had provoked it. Regular skirmishes with the Bagratid realms of the west soon became commonplace, but neither side was yet willing to allow war to break out. However, by the 1250s, relations had become more hostile, and the Mongol Great Khan sent his brother Hulagu to subdue the Bagratids once and for all.
The Mongol army reached Arbela by January 1254 and proceeded to lay siege. By March the city was starving and was ready to surrender, but changed its mind when reinforcements arrived in the area on the 20th. The Mongols hurried to confront the relieving army, allowing much-needed supplies to be smuggled into Arbela.
Battle was joined on the 22nd, and both sides suffered devastating losses. Although the Mongols forced their opponents from the field of battle, they were left with so few able-bodied warriors that they found themselves unable to resume the siege. On the 30th therefore the governor of Arbela, along with the Bagratid kings of Media and Asuristan, agreed to pay tribute in return for the Mongols retreating to the old frontier.