[For the first post on this topic, click here.]
Another potential use of the American nuclear arsenal that appears to be off the table is counter-proliferation. In other words, US leaders are not willing to use nuclear weapons to stop even the worst adversaries from acquiring nuclear technology, despite the high stakes involved.
That situation is not unique to the post-Cold War era. In the early days of the US-USSR confrontation, the United States had substantial nuclear superiority. While many practical considerations restrained US decision-makers--for example, the high risk of a Soviet invasion of Western Europe--other constraints overshadowed any cold-blooded calculations about what it might take to stop the Soviets from expanding their nuclear arsenal. Americans feared the nuclear cataclysms that the future might bring if the USSR were to acquire hydrogen bombs, submarine-launched nuclear missiles, or MIRVed warheads. However, under no circumstances did US leaders think that the prevention of hypothetical carnage, however great it might be, justified the actual carnage that a disarming first strike would create.
The horror of any nuclear attack, even if it were limited to Soviet military targets, overruled any argument for a disarming strike. American leaders did not want to be the executioners of millions of people, even if it might prevent an even greater cataclysm in the future. No one wanted to argue the merits of one apocalypse over another.
Instead, US decision-makers focused on how to reduce the risk of any nuclear war. Crisis management and diplomacy, not pre-emptive strikes, became the tools of choice.
Since the Cold War, the United States has had several opportunities to prevent enemies--Iran, North Korea, and Libya--from gaining nuclear technology, through the most brutally direct methods possible. Regimes that described the United States as the central engine of all evil in the world were apparently eager to acquire the most dangerous weapons in the world. Still, American leaders did not see any justification for a pre-emptive nuclear strike, even if these regimes were powerless to respond.
In other words, humanity operates at some threshold in US foreign policy. A suitcase bomb, built by the Iranian government and smuggled into an American city, is almost too horrifying to envision. Nevertheless, to prevent that scenario, American leaders were not willing to kill millions of Iranians.
The US might credibly threaten conventional war, as the Clinton Administration did in 1994 against North Korea. Nuclear weapons, on the other hand, seem to be a poor tool for counter-proliferation. Not even the threat appears to be useful.