After a massive buildup of nuclear arms, the U.S. and Russia over the years have been reducing the number of weapons ready to fire.

Russia isn't the scary adversary it was during the Cold War. Threats posed almost entirely by Russia are being replaced by new fears—not necessarily associated with any particular country. Money not used for maintaining the weapons can be used for other types of defense.

"We expect to be surprised and so we have to have capabilities that would deal with a broad range of threats," says J.D. Crouch, Assistant Secretary of Defense.

The good news is that with the perceived threat between the U.S. and Russia diminishing, so are the number of warheads deployed—called "strategic" warheads. Under the terms of international agreements, the U.S. has 5,949 strategic warheads. Russia has 5,858, according to the Arms Control Association, an independent organization that keeps tabs on the arsenals.

In 1990, the U.S. had 10,563 strategic warheads, and the former Soviet Union had 10,271. At the Texas summit with Russian President Vladimir Putin in November, President Bush said the number of deployed U.S. warheads would drop to somewhere between 1,700 and 2,000.

Some of these warheads are stored in depots; others are taken apart. Since the fall of the Soviet Union, the U.S. has worked with Russia and other former Soviet republics in an effort to reduce the chance nuclear arsenals end up in the wrong hands. It included digging up missile silos, cutting up missiles and old bombs, taking apart warheads, and improving security at storage sites.

The buildup is a result of the Cold War, when the U.S. and the Soviet Union pursued a policy of nuclear deterrence. The large arsenals were designed to scare the adversary into not trying anything stupid. The problem is, each country has so many weapons spread all over that even after launching a nuclear attack, chances are the other would still be able to retaliate.

That's because each country has a variety of delivery systems to deliver the bombs. Military planners on each side fear the other might strike, so some of these weapons are kept on alert, ready to fire within minutes.

There are free-falling gravity bombs that are carried on aircraft. There are also ballistic missiles, self-propelled weapons that can reach targets thousands of miles away within 30 minutes. They can either be land-based intercontinental ballistic missiles (ICBMs) or submarine-launched ballistic missiles (SLBMs). Cruise missiles are shorter range, and use internal guidance systems—with a computer and radar—to reach their targets.

Still, with so many bombs still aimed at each other, the U.S. and Russia don't exactly trust each other.