What worried him most and what he tried hardest to avoid was a situation that would take him across the nuclear threshold.

But critics say that the Bush administration is, in effect, lowering the nuclear threshold by calling for the development of nuclear weapons that would be easier to use.

That meant that the nuclear threshold was very high.

The clear inference is that the nuclear threshold is becoming robust enough to withstand even fairly overt military challenges.

Since 1970, when the treaty took effect, India, Israel, Pakistan and South Africa are thought surely to have crossed the nuclear threshold by developing weapons.

He said that step would help counteract the current downward spiral toward a lower nuclear threshold.

Most of all, the consequences of crossing the nuclear threshold are, indeed, unthinkable, and the risks of doing so too great.

The flow of foreign technology has also fueled the missile race between Pakistan and India, an ominous development given that the two antagonists have already crossed the nuclear threshold.

Let me emphasize this point: our goal is to raise the nuclear threshold in Europe, not to lower it.