The Bush administration succeeded last week in advancing one of its most radical, dangerous and underdebated policy ideas. Shrugging off objections from a handful of Democrats, both the House and the Senate approved legislative language removing a decade-old ban on research into a new class of “low-yield” nuclear weapons and authorizing $15 million for the study of another category of “robust” warheads designed for underground targets. The administration insists it wants only to pursue research on the new nukes. But even this research will, at a minimum, multiply the incentives for rogue states and rival powers to build nuclear arsenals of their own — a trend that President Bush has rightly defined as the most serious danger of the new century. At worst, the administration will succeed in making nuclear war easier and more tempting, both for the United States and for other powers — an outcome at odds with any reasonable understanding of national security or morality.

The protestations that only research is at stake appear questionable when placed against the nuclear weapons doctrine drawn up by the administration, largely at the behest of a circle of civilian advisers in the Pentagon and White House who for years have been advocating the development of new nuclear weapons. An administration plan disclosed last year called for a three-year process of developing the new arms; another measure approved by Congress last week, to lower the time needed to prepare for new nuclear testing from three years to 18 months, hints at the larger agenda. The new generation of nuclear strategists envisions using “low-yield” weapons — with an explosive force up to one-third that of the bomb dropped on Hiroshima — to attack stockpiles of chemical and biological weapons held by rogue states. The “robust nuclear earth penetrator,” with destructive power at least 70 times that of the Hiroshima bomb, would be intended to reach bunkers buried far underground — like those where North Korea is thought to be producing and storing weapons materials.

Many scientists believe that earth-penetrating nuclear weapons will never be feasible. Current technology limits earth penetration to about 50 feet; a low-yield weapon exploded at that depth would do no harm to a deep bunker while wreaking enormous damage by spewing tons of radioactive rock and soil. A “robust” weapon powerful enough to destroy a bunker 1,000 feet underground, in turn, would kill at least as many people and cause as much damage as a conventional nuke. The administration’s doctrine nonetheless allows for the possibility of using such weapons not only in response to a nuclear attack on the United States but also preemptively against a state thought to be stockpiling weapons of mass destruction. Any such preemptive attack ought to be unthinkable — the harm it would cause this country, and the world, would be catastrophic.

At the moment, the U.S. nuclear arsenal is designed for use only in a situation where the survival of the nation or its closest allies is at stake. That scenario died with the end of the Cold War — but it ought to remain the threshold. The rapid progress of conventional weapons technology offers sufficient means for tackling the problem of deep bunkers and rogue arsenals. By pursuing other options, realistically or not, the Bush administration is feeding the budgets of nuclear weapons labs and the dreams of misguided strategists at the expense of the far more urgent nonproliferation agenda it adopted after 9/11. Congress should return to the issue as the appropriations process proceeds; this is a project that ought to be stopped before it causes real harm.