After President George W. Bush and Russian President Vladimir Putin announced plans to reduce their countries' nuclear warheads, a former Clinton administration official told the Washington Post: "We're breaking what had been an effective sound barrier in the arms control world, which is the 2,000 number. That had always been the holy grail—if you go below 2,000 [the theory went] you'll lose the strategic triad." OK, but what's the strategic triad?

The United States can deliver a nuclear attack by land, by sea, or by air: with land-based intercontinental ballistic missiles, sea-based submarine-launched ballistic missiles, and airborne strategic bombers. That's the triad. Since the 1960s, U.S. nuclear strategy has relied on it to fulfill the pledge of mutually assured destruction: that the country could survive a surprise first strike by the Soviet Union (or other nuclear power) and then respond with a devastating strike of its own. Each leg of the triad is supposed to be large enough to have a deterrent effect. The theory is that a first strike against the United States could not hope to destroy all three legs of the triad at once, and even if two of the three legs of the triad are destroyed, the third can still inflict a retaliatory strike. In addition, having three legs protects against the risk that a new technology (such as a missile-defense system) could threaten the viability of a single delivery system.

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