Going back to the early fifteenth century to tell indigenous people worldwide to use fire arrows against every European ship they see and to organize regular patrols to keep an eye on the ocean at all times. You can also inoculate people against smallpox just by sticking dried smallpox sores into their bloodstream. (The Chinese, Turks, and Africans already know about this at the time.) These two cool techniques could save tens of millions of lives and destroy the historical nightmare the world has been experiencing for the last five centuries before it even begins. Major issues are: organizing people and learning their languages.