Basically, think twice before you act. The song was an answer to the crackdown on student protests in Czechoslovakia, the American antiwar effort, and calls around the Western world during 1968 for a revolution against the system and the status quo.
John is giving his view on the world of possibilities.
I think he was referring to the "then current" social revolution which he equivocates with the 8 prior significant history changing revolutions; Jews vs. Romans, Bolshevik, French, American, Turkish, Cuban, Hungarian, and Irish.