"...a locomotive-size missile that would travel at near-treetop level at three times the speed of sound, tossing out hydrogen bombs as it roared overhead. Pluto's designers calculated that its shock wave alone might kill people on the ground. Then there was the problem of fallout. In addition to gamma and neutron radiation from the unshielded reactor, Pluto's nuclear ramjet would spew fission fragments out in its exhaust as it flew by. (One enterprising weaponeer had a plan to turn an obvious peace-time liability into a wartime asset: he suggested flying the radioactive rocket back and forth over the Soviet Union after it had dropped its bombs.) "
For what its worth, we as a species perfected warfare into an obscene art of self-annihilation, then eventually went "woah this is insane, lets stick to just killing each other with bullets and bombs for a while"
Realistically, it was only scaled back because too many countries had too many of them. Without the threat of MADD, one country would have kept on building crazier nukes and using them.
Also why Russia gets so pissed about the missile shields NATO likes to build closer and closer to its borders.
One of the most important things I learned, having grown up during the cold war, is that the Russians were more afraid of us than we were of them, and had good reason to be. Our military industrial complex thrives during a state of almost-war.