The new nuclear arms treaty that the U.S. and Russia will sign Thursday in Prague may mark a historic return to arms control efforts for the world's nuclear superpowers, but the pact is more a modest step than a major leap along the road to reductions in the world's deadliest weaponry.

Some questions and answers about the new treaty, a replacement for the 1991 Strategic Arms Reduction Treaty, and its place in the bigger picture of U.S. and international security:

Q. What's getting cut and how much?

A. The new START treaty applies to one category of nuclear weapon — those designed to strike from long distances. For the U.S. that means warheads fitted on intercontinental-range missiles aboard Trident submarines, those fitted on similar missiles stationed in underground launch silos in the Midwest, and those carried by two types of bombers: the B-2 and the B-52. The Russian strategic force is weighted more toward land-based weapons than is the U.S. force.

The U.S. has 288 nuclear-armed missiles aboard subs and 450 in underground silos, plus 60 bombers assigned to nuclear strike missions, for a total of 798. The treaty limit is 700, but allows seven years to get there. The Russians won't have to lift a finger to comply, since they already are believed to have fewer than 600.

Q. What about nuclear warheads?

A. Those are counted differently. The agreed limit for both sides is 1,550. In the previous strategic arms agreement, known as the Moscow Treaty of 2002, the limit was set as a range of 1,700 to 2,200. Neither country has said how many it has left, but the U.S. is believed to have between 2,100 and 2,200, and the Russians are believed to have about 2,600.

The way warheads are counted is different in the new treaty, however, so comparisons are imperfect.

The U.S. previously counted an unspecified number of warheads — perhaps as many as 500 — designed to be carried by its bombers. The Russians, however, did not count any in that category because the Moscow Treaty did not require it.

The new treaty takes an entirely different tack: none of the warheads carried by bombers count. Instead, each bomber aircraft counts as one warhead regardless of how many warheads it is capable of carrying. Thus the 60 U.S. bombers count as 60 warheads toward the 1,550 limit, and the Russians' 75 bombers count as 75 warheads. The actual warheads the planes would carry are simply not counted.

Q. Is every other nuclear weapon counted?

A. Hardly. The U.S. has an estimated 6,700 nuclear warheads in reserve and awaiting eventual dismantlement — none of which counts against the 1,550 total. The Russians have a similar number in that not-counted category.

Another category that is not covered is short-range nuclear weapons. The U.S. has an estimated 500, of which about 200 are based in Europe. The Russians have perhaps 2,000, all based in their own territory.

The Obama administration announced on Tuesday that it will push the Russians to negotiate limits for short-range as well as long-range weapons, once the new Strategic Arms Reduction Treaty is signed and ratified by both countries.

Q. Is ratification a sure thing?

A. No, but prospects appear bright at this stage. The Obama administration needs 67 votes in the Senate. More than one committee may hold hearings on the treaty, but the panel that has sole jurisdiction to write a resolution of ratification is the Senate Foreign Relations Committee, headed by Democrat John Kerry, who strongly supports the treaty. The ranking Republican on the committee, Richard Lugar, also has indicated he will support it, although the actual treaty and its detailed annexes are not expected to be submitted to the Senate by the White House for some weeks.

The Russian parliament must also approve the treaty, but as long as the Kremlin supports the pact legislative approval is expected to be a formality.

Upon ratification, the treaty is to remain in force for 10 years.

Q. Why is the treaty important?

A. Not because it dramatically shrinks nuclear dangers. Linton Brooks, a lead U.S. negotiator on the 1991 strategic arms treaty, on Wednesday described the weapons reductions as "modest but not completely trivial." Perhaps more important is the fact that the U.S. and Russian governments have established a bridge to potentially more substantial arms reductions, and have demonstrated to the rest of the world that they are capable of giving up at least some of the weaponry they want others, like Iran, to forgo.