When an Indian seafarer leaves Kochi, Mumbai or Visakhapatnam aboard a merchant vessel, he carries far more than cargo across the ocean. He carries a small part of India’s economic security. When a fisherman pushes his boat into the Arabian Sea, when a container ship enters an Indian port, or when a tanker brings energy from the Gulf, the vastness of the Indian Ocean becomes part of an ordinary Indian life. Yet our national-security imagination has traditionally looked elsewhere—to the Himalayas, the Line of Actual Control and the land borders with Pakistan and China. That is no longer enough. For…