Galerius, "Caesar" under Emperor Diocletian, invaded Mesopotamia, which Narseh had occupied hoping to check his advance. Three battles were fought subsequently, the first two of which were indecisive. In the third fought at Callinicum, Galerius suffered a complete defeat and was forced to retreat. Galerius crossed the Euphrates into Syria to join his father-in-law Diocletian at Antioch. On his arrival at Antioch, Galerius was rebuked by Diocletian who disgraced him for his shameful defeat at the hands of Narseh. Vowing to take revenge, Galerius made preparations throughout the winter of 297 and invaded Armenia with 25,000 men.