In 1296 Alauddin became the sultan after Malika Jan the widow of Jalaluddin and her younger son Qadir Khan left Delhi. He also exterminated the old Balbani and Jalali nobles. The reign of Alauddin Khilji marks the zenith of the power of the Delhi Sultanate. In 1297 he set off for conquering Gujarat. He sent an expedition under Ulugh Khan and Nusrat Khan to Gujarat. On the way Ulugh Khan conquered Jaisalmer. During the plunder of the rich port of Cambay Alauddin's commander Nusrat Khan acquired a Hindu turned Muslim slave Kafur who later on rose to become a great military general and the Malik Naib of Alauddin. After the conquest of Gujarat Alauddin sent an expedition under Ulugh Khan and Nusrat Khan to Ranthambhore. However they were beaten by the Rajputs and Nusrat Khan died. Alauddin went to Ranthambhor and annexed it in 1301. The next expedition was sent to Mewar and after the siege of 8 months he captured Chittor in 1303. The government of Chittor was put in the hands of Khizr Khan, the eldest son of Alauddin. Chittor was renamed as Khizrabad after the name of Khizr Khan. In 1305 Alauddin sent Ain-ul-Mulk Multani for the conquest of Malwa which was placed under the governorship of the latter. By the end of 1305 the whole of Northern India fell into the hands of Alauddin and he directed his attention to the conquest of Deccan.
Between 1307 and 1312 he began the southward expansion of his empire. He invaded Devagiri in 1306-07 AD. The immediate cause for this was unduly long delay in sending the annual tribute. In 1309 the Kakatiya kingdom was attacked and its ruler Pratap Rudra Deva accepted the suzerainty of Delhi and surrendered vast treasures. The next expedition was against Vir Ballala III the Hoysala ruler in 1311. His capital Dwarsamudra was captured. The whole of Deccan was forced to acknowledge the supremacy of Alauddin. His motives were to secure the immense wealth and to force the southern states to accept the suzerainty of the Sultanate. He had to face more than dozen invasions. These invasions started from the end of 1296 and continued upto 1308.The Mongols threatened not only Punjab,Multan and Sindh but even Delhi and the Ganga-Yamuna Doab. This grave crisis compelled him to take strong measures for the protection of the northwest frontier. The 20 years of his rule came to an end with his death on 2nd January 1316 AD.