What if Thera hadn't erupted and the Minoans hadn't been finished off by the Mycenaeans?
What if The Punic Wars were a disaster for Rome and failed to destroy Carthage?
What if the Incas had been more united and drove the invading Spanish out?
What if the 500 Nations hadn't been susceptible to smallpox (has anyone heard of a theory that there was some sort of plague or outbreak right before the Europeans invaded and made conquest easier? I vaguely remember someone saying this but can't remember)?