If there was never a US, the British might have kept their empire for longer (none of that "give up your empire in exchange for our help" during WW2). The Russians might have risen to power regardless (no real US connections). The Germans might have won WW1 (would have started anyway), or it might have dragged on for a few more years. The depression of the 30's might not have been so bad as it was. WW2 might not have broken out (Hitler may not have gained power), but if it had, Germany *or* Russia might have won, depending on how quick Germany was. Japan might have had an empire in the east (No US to stop them), China might have been smaller and less powerful as a result.
There are so many variables, it's pretty much impossible to say what would have happened after the '40s. Just remember that before 1900, the US wasn't really a big player on the world stage. Go from there.
