That is a hotly contested topic in history and political science circles. We've had no nuclear war since their invention, aside from the two initial uses. We've had many low intensity conflicts since then, but nothing like the widescale destruction of the two world wars since their invention. Mutually assured destruction as a deterrent to major war is certainly a controversial topic, but it's hard to argue its had zero impact on the quantity or scale of wars. The cold war very likely would have been a mass hot war of incredible destruction but-for the detente caused by the existence of nuclear weapons.