Nuclear weapons are, in a sense, the ultimate blunt instrument. The immense power and the after-effects of their use make a surgical nuclear strike impossible. As a consequence, they are often kept back, for use only in the most dire of circumstances. But what could bring a country like the United States to the point of using a nuclear weapon on another country? There are two possibilities that rank very high on this scale: Iran and North Korea.
In this case of North Korea, the United States is relying on their nuclear weapons primarily as a deterrent. The North Koreans do have a mature chemical weapons capability, and they also have experimented with biological weapons. This is in addition to a very small nuclear arsenal. North Koreas real risk is two-fold. North Korean leader Kim Jong-Il could decide that he has nothing left to lose, or the cash-strapped North Koreans could sell the nukes to someone else. Preemption of such a move could justify the use of nuclear weapons. Should North Korea use any of its weapons of mass destruction, that, too would result in the use of a nuclear weapon if only to maintain the credibility of deterrence.
Here is one other thing to note here: American policy is to treat chemical and biological weapons as equivalent to nuclear weapons. This is because the United States does not have any real offensive biological warfare capability, and it has been destroying its chemical weapons capability as well. This leaves the United States with only one response to any chemical or biological attack.
The second possibility is Iran, which has a very advanced biological weapons program (in advanced research and testing now) and which also has a mature chemical weapons program, which makes and produces varying chemical weapons. Iran is of great concern because it not only has had generally bad relations with the United States (this is the predictable result of the storming of the American embassy in Tehran back in 1979), but also because it is known to be a major sponsor of Hezbollah, a terrorist organization with a propensity for large-scale suicide attacks (like the bombing of the Marine barracks in 1983). In effect, Hezbollah can be seen as a delivery system for an Iranian WMD strike. What scares a lot of people about Iran is not that they seek nuclear weapons for deterrence (say, against an attack), but that they could provide WMD to Hezbollah, who then use it on an Israeli or American target.
This is one case where not using nuclear weapons would be seen as an act of weakness. The targets would probably be extensive. Iran has at least four major chemical weapons facilities, at least one biological weapons facility, plus the nuclear research program, which includes enrichment facilities in at least three locations. A limited strike would still push the death toll into the hundreds of thousands, if not millions, and a limited strike would risk missing some of the Iranian systems. The only way to be sure Iran would be incapable of launching a second attack would be to wipe Iran off the map.
The other instance in which nuclear weapons could be used would be the deflection of an incoming asteroid or comet. This is not as theoretical or speculative as it might sound. Earth has recently had some near-misses and scares (asteroid 2004 MN4 was one of the most recent in the news, another was 2004 FH).
The short version from looking at these examples is that nuclear weapons are not going to be used lightly by the United States. Only in cases where the United States or an ally is attacked first with WMD, facing imminent attack with WMD, or facing serious damage or extinction from impact from an asteroid or comet would the use become a highly possible event. Such a situation is the stuff nightmares are made of. Harold C. Hutchison (firstname.lastname@example.org)
What would warrant the use of nuclear weapons? This is a huge question, and one worth considering carefully. There are major implications to the use of nuclear weapons. What would today be considered a smaller nuclear weapon (estimates of the yield range from 13 to 20 kilotons) were the same type of weapon that killed 80,000 people outright (60,000 more died of radiation sickness) in the first combat use of nuclear weapons in 1945.