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...

Quite justified in my opinion.

You mean carry out a coup and/or invade?

No, when a government collapses it is not uncommon for a neighboring country to provide structural assistance in establishing a new government.

This only really works with a UN declaration/international support. Having historically friendly ties makes this a better proposition.