He had announced a general relief from taxes to all citizens of Herat when he heard the news that Ulugh Beg crossing the Amu Darya with 90,000 troops. He hastily made his way towards the Tarnab through the Sanjab Pass. The two forces met at Tarnab and after a brief engagement, Ulugh Beg decisively defeated Ala-ud-Daulah Mirza who fled towards Mashad and then towards Quchan in the protection of his brother Abul-Qasim Babur Mirza.