Bakhtiyar Khalji's Tibet campaign
Bakhtiyar Khalji, the general of Qutb ud-Din Aibak, launched a campaign to invade Tibet from March to June 1206. Tibet was a source for horses, the most prized possession of any army, and Khalji was keen to control the lucrative trade between Tibet and India.
Source: Wikipedia — Bakhtiyar Khalji's Tibet campaign (CC BY-SA 4.0)