Doing my first playthrough as the Soviets, and I'm confused how to crush ungrateful Warsaw Pact rebellions: I can't send volunteers, expeditionary forces, or declare war in a puppet's civil war. Can I only send lend lease equipment and hope the communists win?

Also: What triggers these rebellions/civil wars? Events, NF and/or low Marxist-Leninist support? i.e. do I need to keep an eye on (and boost) Marxist-Leninist support in my puppets?