Under Rajaraja Chola and his son Rajendra Chola, the Cholas became dominant in the 10th century and established an empire covering most of South India and Sri Lanka. The empire had strong trading links with the Chinese Song Dynasty and southeast Asia. The Cholas defeated the Eastern Chalukya and expanded their empire to the Ganges. They conquered the coastal areas around the Bay of Bengal and turned it into a Chola lake. Rajendra Chola improved his father's fleet and created the first notable marine of the Indian subcontinent. The Chola navy conquered the dominant Southeast Asian power, the Srivijaya Empire, and secured the sea trade route to China. Cholas exacted tribute from Thailand and the Khmer Empire. The latter half of the 11th century saw the union of Chola and Vengi kingdoms under Kulottunga I. The Chola emperor decisively repulsed an invasion by the Western Chalukya king Vikramaditya VI, who had tried to interfere in Chola politics by installing his puppet, and their defeat of him led to their annexation of Gangavadi and Konkan regions. Vikramaditya VI was confined to his own dominions north of the Tungabhadra. The Chola empire remained formidable during the reign of Kulottunga and maintained its influence over the various kingdoms of Southeast Asia like the Sri Vijaya empire. According to historian Nilakanta Sastri, Kulottunga avoided unnecessary wars and had a long and prosperous reign characterized by unparalleled success that laid the foundation for the well being of the empire for the next 150 years.