re: If you were randomly sent to the past, would you be able to make a historic difference?

I could go back 2200 years ago, invent the stirrup, gun powder, fire arms, artillery, and give whatever kingdom would hire me the most powerful cavalry and siege equipment in the world. The stirrup absolutely revolutionized mounted combat by creating a far more stable firing platform for archers as well as greatly increasing the amount of force a horse-drawn melee combatant could transmit. These innovations allowed for the shocked cavalry charge that dominated Eurasian combat for centuries. It was the biggest single innovation in warfare until cannon brought down the walls of Constantinople.

I could also bring back my knowledge of geography, sunstones, and wayfinding via the stars to allow a pre-columbian civilization early access to the New World.