You just have to be better then them. Its easy if your military units are way too strong for them they ask peace. I focus on just 3 cities till i get tanks If you get the technological superiority you can just defend with archers and build a templar army. With this army you can easily destroy any incoming army that will be for 90% swordsmen expecialy if you bbought road. Then you should plan to finish leonardo's workshop as you discover combustion to get all your archers become musketeers but more than anything you get all your infantry and chivalries promoted to tank armies. Proceed to conquer your worst enemy capital (or his best cultural city) and his other cities will join you due to superior culture but the god thing is everyone will ask for peace The point is focus on tech to get first to tanks and then first to advanced flight then you can get any victory you like.