The Week did an interesting article called: “Why are all of America’s nuclear missiles aimed at Russia?”
The article points out that:
“We have 450 active ICBMs, but because of geographical constraints, they can really only be used to attack Russia.”
(Note: Actually there are more if you consider they carry MIRV’s. Modern ICBMs typically carry multiple independently targetable reentry vehicles (MIRVs), each of which carries a separate nuclear warhead, allowing a single missile to hit multiple targets.)
“Due to the location of missile silos* and launch trajectories, to hit targets in East Asia or even the Middle East, American missiles would first have to fly over parts of Russia.Needless to say, nuclear missiles streaking over Russian territory would trigger alarms and likely a retaliatory attack.”
The article suggests that “to survive an attack, ICBMs must be launched within seconds, minutes at most, leaving little time to verify a false warning. As it stands, only Russia has a nuclear arsenal large enough to even consider attacking American ICBMs, but with the Cold War over, there is little political incentive for the Russians to initiate Armageddon”
Not to worry the article says:
“Just one of the U.S. Navy’s 14 Ohio-class nuclear submarines can hold as many as 288 warheads (although they are only permitted to carry 192, still more than enough to level cities and kill tens of millions).”
Source: http://theweek.com/article/index/255257/why-are-all-of-americas-nuclear-missiles-aimed-at-russia).
* The current US force consists of 450 Minuteman-III missiles in missile silos around Malmstrom AFB, Montana; Minot AFB, North Dakota; and F.E. Warren AFB, Wyoming.