50 to 100 KT airbursted over a city is enough to end that city...
600 KT and higher would be for hardened, strategic, targets. A 600 KT warhead airbursted over a city.
The peackeeper missles I believe still pack warheads in the megatonnage range...and ten of them. I think there are a 100 active silos for peacekeepers.
I think the Iranians are working on a fission device...so the yields are much, much, lower than the thermo nukes we have in our arsenal...a ten kiloton nuke (Hiroshima style) nuke...detonated over Washington is still a bad day, but can be recovered from.
The ABM systems is betting that it will be some third world crackpot's poorly built delivery system that sends the fission bomb at us, which our system could handle.
If the Russian go for a nuke fight..again, MAD, it's all over for everyone. And that is never going to happen.
The Peacekeepers (MX) are out of service. The last one was retired this year but all 50 are being stored indefinitely. They used to be armed with 10 W-87 Warheads with a yield of 300 kilotons per warhead. The 500 W-87 warheads will now arm our 500 Minuteman III ICBM's with one warhead per missile. The W-87 can be upgraded to 475 kliotons by putting an uranium tamper on the second stage.