4 ideas
Histories written from land-based imperial capitals like Delhi, Baghdad or Beijing treat the sea as a boundary and coastal peoples as peripheral. Starting from the ocean instead reveals ports, merchant networks, seafaring communities and naval powers as primary drivers. It also shows that cultures separated by water were often more connected than neighbours separated by land.
The monsoon reverses direction seasonally, blowing roughly from the southwest in summer and from the northeast in winter. This let sailing ships cross the ocean one way and return months later. Because ships had to wait for the wind to turn, traders stayed in foreign ports for long periods, which seeded settled merchant diasporas and cultural transfer as well as trade.
Around 1025 CE, the Tamil Chola king Rajendra I sent a fleet across the Bay of Bengal. It sacked a string of ports belonging to Srivijaya, the Sumatra-based thalassocracy that controlled the Malacca and Sunda straits and taxed the shipping passing through them. The expedition was about access to trade routes toward China rather than territorial conquest, which makes it a rare case of a premodern Indian state projecting naval power overseas.
Before the Portuguese arrived around 1500, Indian Ocean commerce was contested and sometimes violent, but no power systematically tried to own the sea lanes. This converted maritime trade from a multi-party marketplace into a protection racket run by whoever held naval superiority.