[ WP ] Earth is at war with an alien race that has conquered most of the world . You are sent back in time to try and change history and shape Earth into a planet that can defend itself against the aliens .

It'd have to be a time before the dawn of civilization when humanity is too weak and underpopulated to prevent me from conquering them with modern weapons and technology. Once I've conquered every living person in that time, I'll establish myself as emperor of the world or I'll form a global republic idk. 
 
 People in the Stone Age had about the same intellectual capacity as people now. Still, new skills and knowledge are n't learned overnight. It would take numerous generations for everyone to understand how to use the technology I've brought back with me, to fully integrate into the pan-global culture I helped form, and all the speak the same language. 
 
 I'd have the opportunity to prevent climate change by forbidding the use of fossil fuels or at least so much of it. Encouraging the use of non-polluting energy resources. 
 
 I'd also have to leave behind a message for my descendants so that the world never forgets why I changed history in the first place. Humanity would be more advanced because of me and still have thousands of years to catch up and become a match for the aliens.
