Easily: thanks to the development and maintenance of the US nuclear forces, the US will never enter into a direct military conflict with any other first-world country with nuclear weapons of its own. Thanks to the doctrine of mutually-assured destruction, what could have easily been a massively destructive total war between two superpowers was restricted to a half-century of posturing and proxy warfare, all of which killed at least an order of magnitude fewer people than would have died in a direct, total war.

No, it's not warm and fuzzy to think about things in this way, but we're not children and we don't have the luxury of idealistic naivety.