I worry about stuff like "make peace for 100 years" -- because it's hard to know who's responsible for that (it takes everyone make peace). But, more importantly, if you're playing multiplayer, you just need one person to start a war.
You would have to do something like instigate peace between two rival nations. Like a Palestine VS Israel thing. Or impose sanctions on a country that's at war, causing them to go peaceful. These kinds of things multiple times. And, most of all, stay out of conflict yourself -- by having allies who love you, and enemies who would have too much to lose by attacking you.
I think you'd need a few more options, diplomatically. As well as temporary locked alliances -- especially in the modern age, especially for democracies. "Sorry Sire, but we will not let you attack France. We just negotiated peace and will not let you smear Germany's good name!"