The second Pratihara king to defeat Dharmapala and drive away his protege from Kanauj was Nagabhatta I. The following points explain the significance of Nagabhatta I's victory over Dharmapala:

Background:
- Dharmapala was the ruler of the Pala dynasty which had established its supremacy over northern India, including Kanauj, in the mid-8th century.
- However, Dharmapala faced a challenge from the Pratiharas, a Rajput clan that originated in Rajasthan and gradually expanded its power in the Gangetic plains.
- The first Pratihara king to confront Dharmapala was Nagabhata I's father, Vatsaraja, who defeated Dharmapala in a battle near Kanauj in 783 CE.

Nagabhatta I's reign:
- Nagabhatta I succeeded Vatsaraja as the ruler of the Pratihara dynasty in 805 CE.
- In 815 CE, he led an army against Dharmapala and defeated him in a fierce battle near Kannauj.
- Dharmapala's protege, Devapala, who had been appointed as the governor of Kannauj, was also expelled from the city.
- Nagabhatta I's victory over Dharmapala marked a significant shift in the balance of power in northern India, as the Pala dynasty's influence declined and the Pratiharas emerged as a major force.

Impact:
- The defeat of Dharmapala and Devapala weakened the Pala dynasty's hold over northern India and paved the way for the resurgence of other regional powers.
- The Pratihara dynasty, under Nagabhatta I and his successors, went on to establish a vast empire that extended from present-day Rajasthan to Bengal and played a key role in shaping the political and cultural landscape of medieval India.

Conclusion:
Nagabhatta I's victory over Dharmapala was a significant event in Indian history as it marked the beginning of the decline of the Pala dynasty and the rise of the Pratiharas. It also highlights the importance of regional powers in shaping the political dynamics of medieval India.