The Mehranids returned to prominence in the 12th century, when their lands were part of Bagratid Media. For many years Rey was in the frontier region between the Bagratids and the Mongols, and the Mehranids built up a formidable army to oppose the frequent Mongol raids across the border. After the Arbela campaign of 1255, in which the Mongols overran the eastern Bagratid lands before withdrawing, the Mehranids came to rule over Media itself, and expanded in the following years to fill the political vacuum as their neighbours weakened.