this is sort of true. The number of interstate wars (state vs state) has dropped drastically since the end of World World 2 They were almost extinct for the past few decades, besides a few key events like Iraq, Afghanistan, and Ukraine.

However what we got instead are intrastate wars, better known as civil wars. And my god we have so many of them, and they have been on the rise. America loves using her hegemonic power to foment civil conflict, and those wars aren't any less brutal. Honestly they are usually worse.