THIS has been a remarkable time for the Obama administration. After a year of intense internal debate, it issued a new nuclear strategy. And after a year of intense negotiations with the Russians, President Obama signed the New Start treaty with President Dmitri Medvedev in Prague. On Monday, the president will host the leaders of more than 40 nations in a nuclear security summit meeting whose goal is to find ways of gaining control of the loose fissile material around the globe.

New Start is the first tangible product of the administration's promise to "press the reset button" on United States-Russian relations. The new treaty is welcome. But as a disarmament measure, it is a modest step, entailing a reduction of only 30 percent from the former limit - and some of that reduction is accomplished by the way the warheads are counted, not by their destruction. Perhaps the treaty's greatest accomplishment is that the negotiations leading up to its signing re-engaged Americans and Russians in a serious discussion of how to reduce nuclear dangers.

So what should come next? We look forward to a follow-on treaty that builds on the success of the previous Start treaties and leads to significantly greater arms reductions - including reductions in tactical nuclear weapons and reductions that require weapons be dismantled and not simply put in reserve.

But our discussions with Russian colleagues, including senior government officials, suggest that such a next step would be very difficult for them. Part of the reason for their reluctance to accept further reductions is that Russia considers itself to be encircled by hostile forces in Europe and in Asia. Another part results from the significant asymmetry between United States and Russian conventional military forces. For these reasons, we believe that the next round of negotiations with Russia should not focus solely on nuclear disarmament issues. These talks should encompass missile defense, Russia's relations with NATO, the Conventional Armed Forces in Europe Treaty, the Intermediate-Range Nuclear Forces Treaty, North Korea, Iran and Asian security issues.

Let's begin with missile defense. Future arms talks should make a serious exploration of a joint United States-Russia program that would provide a bulwark against Iranian missiles. We should also consider situating parts of the joint system in Russia, which in many ways offers an ideal strategic location for these defenses. Such an effort would not only improve our security, it would also further cooperation in dealing with the Iranian nuclear threat, including the imposition of consequential sanctions when appropriate.

NATO is a similarly complicated issue. After the cold war ended, Russia was invited to NATO meetings with the idea that the country would eventually become an integral part of European security discussions. The idea was good, but the execution failed. NATO has acted as if Russia's role is that of an observer with no say in decisions; Russia has acted as if it should have veto power.

Neither outlook is viable. But if NATO moves from consensus decisions to super-majority decisions in its governing structure, as has been considered, it would be possible to include Russia's vote as an effective way of resolving European security issues of common interest.

The Russians are also eager to revisit the two landmark cold war treaties. The Conventional Armed Forces in Europe Treaty enabled NATO and Warsaw Pact nations to make significant reductions in conventional armaments and to limit conventional deployments. Today, there is still a need for limiting conventional arms, but the features of that treaty pertaining to the old Warsaw Pact are clearly outdated. Making those provisions relevant to today's world should be a goal of new talks

Similarly, the 1987 treaty that eliminated American and Soviet intermediate-range ballistic missiles was a crucially important pact that helped to defuse cold war tensions. But today Russia has neighbors that have such missiles directed at its borders; for understandable reasons, it wants to renegotiate aspects of this treaty.

Future arms reductions with Russia are eminently possible. But they are unlikely to be achieved unless the United States is willing to address points of Russian concern. Given all that is at stake, we believe comprehensive discussions are a necessity as we work our way toward ever more significant nuclear disarmament.

William J. Perry, a senior fellow at the Freeman Spogli Institute for International Studies, was the secretary of defense from 1994 to 1997. George P. Shultz, the secretary of state from 1982 to 1989, is a distinguished fellow at the Hoover Institution.

Hendrik Hertzberg writes that the end of the Cold War and the coming of global warming have brought about increased support for nuclear power, even among some environmentalists (The Talk of the Town, March 22nd). But many of us who work on nuclear-proliferation issues are dismayed by the growth of nuclear energy. Expanded nuclear power in industrial countries will inevitably mean expanded nuclear exports to less developed countries as manufacturers try to recoup their investments in a limited domestic market by selling abroad. It can be shown statistically that countries that receive nuclear assistance are more likely to build nuclear weapons, especially when they perceive threats to their security. India, Pakistan, and Israel started their nuclear programs with the importation of research reactors carrying peaceful-use requirements; with help from other countries, they were able to then realize their desire for weapons. Iran appears to be heading in the same direction. Given the documented interest in nuclear materials of terrorist organizations like Al Qaeda, and given the questionable assumptions that nuclear-energy economics is fraught with, it makes little sense to push nuclear power at a time when protections against proliferation are still so problematic. Improved energy efficiency is a safer, greener, and cheaper alternative.

With an April 8 date set for the United States and Russia to sign a new nuclear arms reduction treaty, each country is preparing to cut their deployed weapons by about 30 percent. That caps each side at 1,550 nuclear warheads and bombs and 700 deployed strategic missiles and bombers.

The pact, which needs approval by the U.S. Senate and Russian Duma, is the culmination of a year's worth of often tumultuous negotiations. It's also an important step in President Obama's push for a nuclear-free world, an idea that was given a roadmap during a 2006 conference at Stanford's Hoover Institution. The conference, which was convened by former Secretary of State George Shultz and Stanford physicist Sidney Drell, resulted in a Wall Street Journal op-ed in January 2007 calling for a world without nuclear weapons.

The piece was written by Shultz, a professor emeritus at Stanford's Graduate School of Business and a distinguished Hoover fellow; William Perry, President Clinton's defense secretary and an emeritus senior fellow at Stanford's Freeman Spogli Institute for International Studies; Henry Kissinger, who served as secretary of state in the Nixon and Ford administrations; and Sam Nunn, a former chairman of the Senate Armed Services Committee and CEO of the Nuclear Threat Initiative.

President Obama mentioned the four men in a March 26 statement announcing the new treaty, noting their support for more assertive action in reducing nuclear weapons.

David Holloway, a professor of international history and faculty member at FSI's Center for International Security and Cooperation, participated in the Hoover conference and has analyzed the steps taken to shrink the world's nuclear stockpile.

Holloway, author of Stalin and the Bomb: The Soviet Union and Atomic Energy, 1939-1956, spoke with the Stanford News Service about the latest pact between the United States and Russia, and what the prospects are for further reduction of nuclear weapons.

Put the treaty in context. How significant is it?

You could say it's a small step in an important process. In the 1980s, there were about 70,000 nuclear weapons in the world. Most were owned by the U.S. and the Soviet Union. Now there are about 22,000 nuclear weapons, 90 percent of them owned by the U.S. and Russia. A number of those weapons are slated for dismantling, but it takes time to do that. Meanwhile, the feeling is that it's better to regulate the US-Russian nuclear relationship by treaty, so that it does not develop in an unpredictable way or a way that causes instability in the relationship.

This treaty reduces only the number of deployed warheads and nuclear delivery systems. What will happen to those weapons?

Some missile sites will be closed down and the warheads will be put into storage. The treaty apparently won't commit either side to dismantling the warheads. It only moves them from deployment. But cutting the number of delivery systems is important because if you don't have the missiles or bombers to launch the warheads, then the warheads aren't much use.

Is there a system in place to keep each country in compliance with the treaty?

Each country has the capacity to monitor the other side's compliance with the treaty. There are satellites that can see what the other side is doing; there are arrangements for the electronic monitoring of test flights and so on; and there are exchanges of inspectors. The two countries have considerable experience of cooperation in this area.

The treaty does not restrict America's plans to build a missile defense shield in Europe. But explain the tensions between Russia and U.S. over that issue.

This was probably the most difficult part of the negotiations. The Russians were eager to get limits on American defenses against ballistic missiles, and the U.S. was very reluctant to include them in this treaty. The Russians are worried what the effect of defense systems would be on their ability to retaliate in the event of an American first strike - as improbable as that is.

Despite that tension, the Obama administration has said it wants to "reset" U.S.-Russian relations. Does this treaty help?

The treaty makes great sense in terms of that agenda. It's an affirmation of Russia's position as a nuclear superpower, and it gives the Russians some assurance that they will maintain the status of an American partner in this area.

What the United States wants is help on issues like Iran and Afghanistan: making sure we can get supplies across Russia to Afghanistan and persuading Russia to continue putting pressure on Iran to back away from making nuclear weapons.

The treaty will have to be ratified by the U.S. Senate. How do you expect that to play out?

The mood in Washington isn't very bipartisan at present, of course. And there are many people who think: why should we have an agreement with the Russians? We're stronger; they're weaker. We shouldn't limit our own flexibility by negotiating agreements. That was a strong view in the Bush administration - that arms control is a bad thing and it only limits our freedom of action. And the issue of missile defense systems will be a contentious issue. There are people who want to see absolutely no restrictions on our defenses against ballistic missiles, whereas that is one of the goals of Russian policy.

How does this treaty fit in with concerns that unstable countries and terrorist groups might get their hands on nuclear weapons?

The Russians aren't about to blow us up, and we're not about to blow them up. The real fear is that other people will get hold of nuclear weapons. In the Obama administration's view, this treaty is part of a single effort to create a tough nuclear regime where states that have nuclear weapons are taking steps toward getting rid of them. And at the same time, the mechanisms for preventing new states - and in particular terrorist groups - from getting hold of nuclear weapons or the materials to make them are being strengthened.

Under the nuclear nonproliferation treaty, which entered into force in 1970, states that have nuclear weapons are obliged to pursue nuclear disarmament, while the states without them have promised not to acquire them. So if you want to strengthen this nuclear regime and make it harder for other states and terrorist groups to get nuclear weapons, then those with the nuclear weapons need to be moving toward zero. That's a key element in the administration's policy. The judgment is that a discriminatory regime is not viable in the long run.

What's the likelihood that we'll get to world free of nuclear weapons?

The president laid that out as a goal, and he said it probably wouldn't happen in his lifetime. Nobody can say that we can get to zero in say 20 years, but we do know what the first steps should be on such a path, and this treaty is one of them.

Before the world could get to zero nuclear weapons, there would have to be certainty that nobody could break out and say, "I've got lots of nuclear weapons, so you better listen to me."

The goal of zero is a vision, but I think it's an essential one because it gives you a sense of the direction you should go in.

What are the next steps Russia and the U.S. will take to reduce their nuclear stockpiles?

It's not clear. There is no agreement to have a further round of talks, but I very much hope there is one. There could be further negotiations on the reduction of strategic forces, but it seems more likely that talks might focus on the possibilities of cooperation in ballistic missile defense and/or on tactical nuclear weapons - the shorter-range systems that are not covered by the new treaty.