I'd send Chuck Norris back to the old Roman empire. He'd stop the invasions before they started, then head on back to Rome to knock heads until they pulled theirs out of their asses and started acting like an empire rather than a bunch of children looking for amusement. Then he'd build a boat, go to America, and give the locals diseases so that they would have immunities by the time Europeans arrived.
That would at least put off the dark ages for a few centuries, while ensuring that the American civilizations would survive European contact instead of being steamrolled and thrust into cycles of poverty and economic/political/environmental collapses. All those centuries of lost productivity would be saved, and my alternate-future self would have a flying car that burns water and emits rainbows.