or even those who are friendly to us. It's a matter of political stability and sanity. As screwed up as the Soviet Union was, it was until 1991 a stable government ruled by sane men. They certainly weren't friendly to the US, but they were sane enough to realize that starting a nuclear war was a one way ticket to destruction. The same is true for China. As the Soviet Union dissolved, there was a lot of legitimate concern about political instability and who would end up in control of the nukes. Fortunately, the matter was resolved in pretty short order.

As to Tom's question about countries like Brazil or Australia having nukes, it isn't just that these countries are friendly to us. They're examples of governments mature and stable enough to take nuclear weapons seriously. Other countries, such as North Korea or Iran, don't fall into this category. They may well use the weapons. Israel is widely known to have nuclear weapons even though they don't admit it. During the darkest days of the 1973 Yom Kuppir War when it looked like they might be overran, they reportedly pulled their weapons from the bunkers and made ready to use them. Fortunately, the military situation changed and that didn't happen. Likewise, a few years ago, India and Pakistan were threatening to hit each other with nuclear weapons in their ongoing dispute over the Kashmir region. Fortunately, cooler heads prevaled and they backed down. Since then, both nations have made big strides to reduce tensions. In both examples, the nations displayed the maturity to back away from the nuclear brink.

One interesting fact about nuclear weapons is how little military utility they really have. The biggest value of them (and the reason why countries like North Korea want them so badly) is the deterant effect they have. Suppose North Korea had even a small number of nukes (which they may already have) and the means to deliver them (they're working on it), especially to even outlying American cities like Anchorage or Honolulu. Would we be willing to risk them attacking those cities should we try to oppose them invading South Korea? The concept is called nuclear blackmail and it works so long as there is no defense against their delivery means.

The physics of nuclear weapons are well understood. Achieving the technology sufficient to make nuclear weapons is still challenging, but the Kahn network ran out of Pakistan made it easier for several nations to acquire these weapons, despite all the nonpoliferation treaties that have been in place for years.