The U.S government today released a precise accounting of its strategic nuclear forces, something it is required to do by treaty, and it's worth a careful read.
The world now knows that, by February of 2018, the U.S. will have approximately 400 intercontinental ballistic missiles, down from 450; 240 submarine-launched ballistic missiles, down about 50; and 60 nuclear-capable heavy bomber fighters (B-2As and B-52Hs), converting 30 B-52s to a non-nuclear role.
Since most of the nuclear payloads contain multiple warheads, the U.S. must also disclose the number of strategic nuclear weapons it will maintain on an alert status. As of 2018, that will be 1,550.
The good news: the number of viable nuclear warheads in the world will go down. President Obama has prioritized nuclear arms reduction, and the Senate in 2010 ratified a treaty with Russia that reduces to 700 the number of nuclear delivery vehicles. (The U.S. can keep an extra 100 platforms in storage.)
The timing is interesting, of course, but the decisions to reduce certain types of weapons is even more interesting. Of the three "guns," the silo-based ICBMs are the oldest, the least efficient, and operated by missileers who have had well-publicized troubles with cheating and morale. But the cuts to that "leg" of the triad are much smaller, proportionally, than the cuts sustained by the Air Force's nuclear fighter wings and the Navy's ballistic missile submarines.
It may well be that the Obama administration decided to boost the confidence of the missileers, but the plan to keep most of the ICBMs might serve another purpose. It will require future administrations to cut the ICBM force more heavily, while giving nuclear planners more time to adapt the new set of platforms to existing targets. The composition of the nuclear force is unclassified; virtually everything else about nuclear war remains a state secret.
For example: cutting the number of strategic warheads will force a big change to the Joint Strategic Capabilities Supplement annex to the current nuclear war plan, OPLAN 8010-12 Strategic Deterrent and Force Employment, as well as to the exercises used to test forces on the plan and the intelligence that guides it. Also, the U.S. maintains a stockpile of battlefield nuclear weapons, which have "yields," or explosive power equivalent to as little as 300 tons of TNT. Most are kept in storage in bunkers across the world. Their locations, types, and numbers are classified, although the U.S. admits to a force of at least 500 "battlefield" weapons.
The U.S. also keeps a big reserve of nuclear weapons material and equipment — the "nuclear strategic reserve," which, while disassembled, do not count towards any of the treaty's red lines. As of 2010, the reserve stock was equivalent to 2,800 weapons. These are intended (in nuclear doctrine) to hedge against strategic surprise, but the number is probably significantly higher than it needs to be, particularly if the classified target countries (China, North Korea, Russia, Iran, Syria) are no longer formally chartered enemies.
Though President Obama has changed the policy undergirding the employment of nuclear weapons, the exact language of the war plan, as well as the thresholds that might trigger the consideration for the use of nuclear weapons, remain classified, even though there is considerable ambiguity built into the precision. It is not known, for example, how flexible the U.S. can be in response to a conventional attack from a non-nuclear country, like Syria. After 9/11, a "WMD hedge" was built into the war plan, too. The U.S. does not rule out using nuclear weapons to respond to a terrorist attack from a non-state actor.