ABM Treaty
The 1972 agreement in which Washington and Moscow promised to stay defenseless against each other's missiles, and which the United States walked out of in 2002.
The Anti-Ballistic Missile Treaty is the clearest example of a rule that sounds insane until you follow the logic: two governments formally agreeing not to protect their own populations from nuclear attack. Nixon and Brezhnev signed it in Moscow on May 26, 1972, and for thirty years it was the load-bearing wall of nuclear arms control.
Why defense was the problem
Deterrence under mutual assured destruction works only if each side is certain it can retaliate. A missile defense system, even a mediocre one, corrodes that certainty in two ways. It tempts its owner to strike first, on the theory that a defense which cannot stop a full attack might well stop the ragged remains of one. And it terrifies the other side into building more warheads, decoys, and delivery systems in order to swamp it, which is far cheaper than the defense itself.
Both superpowers had discovered this the expensive way in the 1960s. The Soviets built a system around Moscow; the Americans, alarmed, planned Sentinel and then Safeguard. Each program prompted the other side to expand its offensive arsenal, so both spent enormous sums to end up less secure. The treaty was the recognition that defense was the destabilizing thing, and that leaving your cities hostage was the price of stability.
What it said
Each side was allowed two defended sites of one hundred interceptors each, one around the national capital and one around an intercontinental missile field. A protocol in 1974 cut that to one site apiece. Nationwide defense was prohibited outright, as was any basis for it: no sea-based, air-based, space-based, or mobile land-based ABM systems, and no giving air-defense systems anti-missile capability.
In practice each side kept one. The Soviets kept theirs around Moscow, and Russia maintains a descendant of it today. The Americans completed Safeguard in North Dakota, declared it operational in the autumn of 1975, and shut it down within months on the grounds that it was not worth the money.
The treaty came bundled with SALT I, the first agreement capping offensive launchers. That pairing was the point: capping defense made capping offense possible.
The long argument, and the exit
The fight never really stopped. Ronald Reagan's Strategic Defense Initiative in 1983 proposed exactly the kind of nationwide shield the treaty forbade, and his administration floated a broad reinterpretation to permit testing, which Congress resisted. In the 1990s the question shifted from Soviet missiles to whether a limited defense against North Korea or Iran was worth the treaty.
George W. Bush answered it. In December 2001 he gave the required six months notice, and on June 13, 2002, the United States left, the only time it has withdrawn from a major arms control treaty. Russia responded by declaring itself no longer bound by START II. The United States went on to deploy interceptors in Alaska and California and, later, systems in Romania and Poland that Moscow insists are aimed at it.
Whether the withdrawal was a mistake is still argued. What is not in dispute is that Russia has cited it ever since as the justification for developing weapons designed to make defenses irrelevant, including hypersonic glide vehicles and a nuclear-powered torpedo. Vladimir Putin has said so explicitly.
The treaty's deeper claim is the uncomfortable one. It held that in a nuclear standoff, vulnerability is not a bug to be engineered away but the mechanism that keeps the peace, and that a state which makes itself safe makes everyone, including itself, less so. Nothing about the physics has changed since 1972. Only the willingness to live with the conclusion has.