For the past decade, the United States has been a model citizen in the world of nuclear nonproliferation. It has not designed, developed or tested nuclear weapons. And it has signed treaties to reduce the size of its nuclear arsenal, the latest of which will cut America's nuclear stockpile by almost two-thirds. These decisions at the end of the cold war heralded the arrival of a less dangerous security environment.

But last week's decisions on Capitol Hill signal a move away from the nuclear stasis of the past decade. Both the Senate and House approved legislative language that repeals a 10-year ban on research into a new class of battlefield nuclear weapons.

The Bush administration is particularly intent on developing a 'robust nuclear earth penetrator' that could be used to destroy underground bunkers. And US nuclear scientists appear ready to conduct studies on a "low-yield" nuclear weapon that could incinerate stockpiles of chemical and biological weapons.

Critics say that such moves may put the United States on a slippery slope to use nuclear weapons again. The only time to date that nuclear weapons have been unleashed was the bombings of the Japanese cities of Hiroshima and Nagasaki in World War II.

Ellen Tauscher, a Democratic Congresswoman from California who sits on the House Armed Services Committee, says "the first one they have coughed up is a thing called the robust nuclear earth penetrator weapon, and it is effectively a weapons system in search of a military agreement to use it. We do know that we have deeply buried and hardened targets out there that we need to defeat, but we have successfully defeated those targets, including in Afghanistan and in Iraq most recently, using a combination of precision bombing, intelligence and special forces. The thought that we would actually use a nuclear weapon tactically is stunning, and this administration appears to have no problem talking about doing that, and the fact that we would begin designing new weapons should be stunning to the American people."

However, those who favor repealing the ban on research argue that the current world situation leaves the United States little choice but to use all of its resources to defend itself. And currently, they say, America doesn't have the right kinds of weapons to destroy hardened underground bunkers that might house an enemy country's leadership or stockpiles of chemical, biological or nuclear weapons. Strategic nuclear weapons are too big and their radioactive fallout uncontrollable, and conventional weapons aren't powerful enough. Only smaller-yield nuclear weapons can get the job done, they contend.

"The difficulty with nuclear deterrence is that you need to hold at risk things that other countries and potential enemies consider to be of value, and over the last decade we have seen potential enemies digging in and putting things they most value underground," says Congresswoman Heather Wilson, a Republican from New Mexico, who chaired a review of US nuclear weapons strategy in the House of Representatives last year and is also a member of the House Armed Services Committee. "We can only hold those things at risk now using very, very large nuclear weapons. And we don't have conventional penetrators that can hold those hard and deeply buried targets at risk," she says.

The move to gain approval to design new nuclear weapons reflects the so-called Bush doctrine, which says America will act, preemptively if necessary, to defend itself from threats posed by countries or terrorists groups with weapons of mass destruction. And leading administration officials have been saying openly that traditional efforts at nonproliferation such as diplomacy and arms control -- have not been working.

In the more threatening security environment of the 21st century, the danger is that without a credible weapon to deter them, America's enemies may feel emboldened, says Daniel Gouré, a former Director of the Office of Strategic Competitiveness at the Pentagon and now a Vice President at the Lexington Institute. "We have seen in the last half dozen years a marked upsurge in proliferation both of weapons of mass destruction and of long-range delivery means," he says. "As a result, there is a certain set of countries, particularly the ones we worry about in the axis of evil, who may actually see a use for these weapons or hope they can get away with a conventional aggression or support for terrorism because of their so-called nuclear umbrella or nuclear shield."

Mr. Gouré adds the United States needs to show it is ready to counter new threats with a new form of deterrence, which relies not on overly powerful, strategic warheads but on smaller, more precise nuclear weapons. "We want to be able to tell potential adversaries that you cannot even bury your capabilities deep enough to hope to avoid direct action or retaliation should that be necessary," he says. "The net result is that we may be able to dissuade some potential adversaries or some particular proliferators from pursuing weapons of mass destruction by the assurance that in a timely manner we can respond with capabilities that can negate their efforts."

Opponents worry that the United States is losing an opportunity to make the world safer. Instead of using its leadership to encourage nonproliferation efforts, Congresswoman Tauscher says the United States is eroding the taboo against using nuclear weapons.

"This is a totally different way of dealing with nuclear weapons," says Congresswoman Tauscher. "It moves it from strategic deterrence --which means I don't want to use it, but this is a threat I can uphold -- to tactical battlefield weapons where you would actually use them. And that is a stunning outcome for the 21st century."

How will the US decision to resume research on new nuclear weapons impact on the world?

Some observers say it sends the wrong message. At a time when the United States wants other countries to give up weapons of mass destruction, America is considering creating new nuclear weapons at home, thereby undermining its commitment to the Nuclear Nonproliferation Treaty, or NPT. John Spratt is a Congressman from South Carolina and a member of the House Armed Services Committee. "The question is: can we move the world in one direction, away from nuclear weapons, particularly tactical, theater, small nuclear weapons that would have lots of portability and applicability early in a conflict, and at the same time ourselves move in a different direction? The underlying premise of the Nuclear Nonproliferation Treaty was that the nuclear powers would have exclusive right to nuclear weapons but that they would over time diminish their arsenals, and we are concerned that this sends the wrong signal to the signatories to the NPT."

Henry Sokolski, a former Deputy for Nonproliferation Policy in the Pentagon during George Herbert Walker Bush's presidency and Executive Director of the Nonproliferation Policy Education Center in Washington, adds "it can't work that we want others to rely less on nuclear weapons, and we should somehow look as though we want to rely more on them. This is a tricky piece of jujitsu in policy."

Proponents of a more robust nuclear posture counter that US actions in nonproliferation, while laudable, haven't much influenced other countries. Congresswoman Wilson says that nations such as Iran, Iraq, Libya, Syria and North Korea, that are believed to have started weapons of mass destruction programs, have not been swayed by US-Russian treaties to reduce the numbers of their nuclear weapons. "Our self-restraint doesn't induce self-restraint in other countries. So I would not expect that the reverse would have much effect on them either."

Before forwarding the 2004 defense budget to the White House for signing, both houses of Congress will convene to forge one document. That compromise bill will most certainly remove restrictions on nuclear weapons research. Will the United States then go ahead and develop new nuclear weapons? That may depend on events overseas and the US response to them.