India, but the two countries have a very complicated and touchy relationship. Sri Lanka's Tamil minority, including the Tamil Tigers, was heavily supported by Tamils in India. However, Tamil Nadu (the Indian state) itself has a fractious relationship and the Tigers repaid India's attempt to send in peacekeepers by assassinating Indian PM Rajiv Gandhi.
Minor correction - Gandhi wasn’t assassinated for sending in peacekeepers. It was for sending in and leaving unchecked a notoriously brutal “peacekeeping” force who terrorized the people they were there to protect: https://en.wikipedia.org/wiki/Indian_Peace_Keeping_Force#War...
