In 1303, a Mongol army from the Chagatai Khanate launched an invasion of the Delhi Sultanate, when two major units of the Delhi army were away from the city. The Delhi Sultan Alauddin Khalji, who was away at Chittor when the Mongols started their march, returned to Delhi in a hurry. However, he was unable to make adequate war preparations, and decided to take shelter in a well-guarded camp at the under-construction Siri Fort. The Mongols, led by Taraghai, besieged Delhi for over two months, and ransacked its suburbs. Ultimately, they decided to retreat, having been unable to breach Alauddin's camp. The invasion was one of the most serious Mongol invasions of India, and prompted Alauddin to take several measures to prevent its recurrence. He strengthened military presence along the Mongol routes to India, and implemented economic reforms to ensure adequate revenue streams for maintaining a strong army.