Why did a Tamil emperor send a navy across the Bay of Bengal in 1025?

In 1025 the Chola Emperor Rajendra I launched naval invasions on Srivijaya in maritime Southeast Asia, leading to the fall of the Sailendra Dynasty of Srivijaya. It was a unique event in India's history and its otherwise peaceful relations with the states of Southeast Asia. Two decades earlier relations had been cordial: a Srivijayan Maharaja constructed the Chudamani Vihara in the Chola port of Nagapattinam in 1006.

Why attack? The reasons are unclear. Srivijaya controlled two major naval choke points, the Malacca Strait and the Sunda Strait, and enforced a naval trade monopoly, compelling ships passing through its waters to stop at its ports or risk being plundered. Nilakanta Sastri suggested that the conflict may have arisen from Srivijayan attempts to obstruct Chola trade with the East, especially China, or from Rajendra's desire to extend his world conquest across the sea, adding prestige to his reign. Another theory points to a quarrel among allies: the Khmer king sought Rajendra's help against Tambralinga, which in turn sought aid from Srivijaya.

How did it work? Indian ships typically sailed east across the Bay of Bengal, but the Chola fleet sailed directly to the west coast of Sumatra. The Srivijayan navy, stationed at Kedah, was unaware of the Chola invasion approaching from the south via the Sunda Strait. The first city attacked was Palembang, the empire's capital.

The war ended in a Chola victory that significantly weakened Srivijaya and broke its maritime monopoly in the region. The Cholas did not establish lasting control over the captured cities, as the campaign involved fast-moving raids and plunder.