I agree with the trumpeteer person. Seriously, the whole world would get too difficult for one person or government to control. There would be rebellions and revolts and what not. But as to what I think about all the choices....
USA-NOW: Well if the Americans built up their forces some more and got better leadership

*cough*, then I guess they could certainly try. But to take on the whole world...No. Just no.
USA-COLD WAR: This was said earlier, but thanks to the USA's standoff with the Soviets, trying to take over the world would have led to too much nuclear conflict.
SOVIET UNION: I don't think so. The people would definietly not enjoy being ruled by Communists, and missing their democracies (if they had them before) they would rise against the Soviets. Too much pressure and worries about rebellions would lead to the Soviets' eventual collapse.
NAZI GERMANY: I think Nazi Germany is overrated in terms of how powerful they were. Sure they were strong, and sure, they almost took over Europe, but as was seen in WWII, the Americans and Brits would prove too much for the Germans.
COLONIAL ENGLAND: Well even though I go for Greece most of the time because of my Greek heritage, I have to hand it to the Brits here. They ruled the seas about half of the time over the last thousand years, and a good navy is vital. I admire the British for conquering India, which even the blessed King Alexander the Great failed to take.
ROMAN EMPIRE: This is where I get angry. I absolutely hate it when people say that the Romans were better than the Greeks, etc. As my grandmother always said, the Romans were the great doers, but the Greeks were the great thinkers. A lot of Roman technology came from Greece, and I don't think many people realize that. But I'm off-topic here. To make a long story short, no to the Romans.
GREECE: As much as I would love to vote for Greece, even I have to admit that the Greeks couldn't possibly have conquered the world. Alexander took over most of the known world, but as was seen, illnesses and revolts crushed him.
THE MONGOLS: Oh boy...Yeah, they were excellent fighters, but I don't think they were that technologically advanced, were they?
