There are already Sociopathic Oligarchs whose fortunes dwarf the GDP of many small countries. You are assuming they'll be challenging superpowers, but they don't need to do that.

All they have to do is conquer a small nation, probably an island, that NOBODY is going to defend beyond a chewing out, and they've been chewed out before.

So then they control a place like Madagascar, or Papua New Guinea, or whatever, and it's doubtful that the rest of the world will go against them, especially when all the Oligarchs in those countries warn their bought and paid for politicians to stay. Besides, a consortium of Trillionaires can buy off any country that objects.