I don't know why any large country would invade instead of just slowly, secretly buying the smaller country and it's power over a decade or two. Take China and Taiwan, or this situation in Ukraine. Even a heavy handed economic hostile takeover would be ignored by the other world powers because no individual politician wants to be responsible for starting the fighting. Eventually you own everything, you've moved a ton of citizens in, you've bought all the politicians with puppet corporations. Then they vote to join your wonderful nation.