How Significant a New START for the U.S. and Russia?

President Obama and Russian President Dmitry Medvedev posed for the cameras in Prague on Thursday before solemnly signing a treaty to reduce by 30 percent their number of active strategic nuclear weapons. Americans over 40 can be excused for recalling Yogi Berra's line about deja vu all over again. They have seen this show each decade since the 1970s. Many college students, however, will be puzzled by what seems a historical anachronism. As a freshman said to me recently: Weren't nuclear weapons a Cold War story?

Today's 18-year-olds, of course, were born after the Soviet Union disappeared.

The Obama administration will tout "New START" as a significant step toward the president's vision of a world free of nuclear weapons. Predictably, critics will counter that it requires minimum U.S. adjustments and essentially ratifies reductions Russians are already making for economic reasons.

Cutting through the spin, four points about the agreement's significance stand out:

First, the treaty returns to the mainstream of superpower arms control that began with President John F. Kennedy. Leaving behind President George W. Bush's unilateralism, this give-and-take agreement mirrors earlier treaties of George H.W. Bush and Ronald Reagan. Such agreements and their verification regimes provide a predictability that dampens the prudent paranoia of national security planners who must consider worst-case "what-ifs."

Second, the agreement is substantively and symbolically significant in the "reset" of U.S.-Russian relations. Without the deep cooperation demonstrated by this treaty, neither country has hope of preventing the proliferation of nuclear weapons, nuclear terrorism and nuclear war. Further reducing these dangers must be a high priority for the leaders of the two nations that continue to hold 95 percent of the world's nuclear weapons and materials.

Third, though the countries' active strategic nuclear weapons stand to be reduced by only 30 percent, the treaty provides credible evidence of each side's seriousness about fulfilling its Non-Proliferation Treaty (NPT) commitments to negotiate toward ultimate elimination. Critics will note that the United States and Russia are allowed to maintain 1,550 strategic nuclear warheads and a much larger number of weapons and weapons-equivalents in their stockpiles. But this misses the point: Consider how numbers in the new START Treaty compare with those at the height of the Cold War. The United States and Russia had more than 68,000 nuclear weapons at the Cold War's peak. In 1991, when the Cold War ended, those arsenals stood at 55,000. New START will reduce deployed strategic warheads to levels almost 90 percent below the high.

Fourth, the most critical issue is the relationship between these U.S. and Russian actions and the behavior of Iran, North Korea, Pakistan and others. How will reductions in U.S. or Russian strategic nuclear arsenals affect Pakistanis' calculations about securing their arsenal from Taliban or al-Qaeda sympathizers in their security services, or the nuclear ambitions of Iran's thugocracy, or Osama bin Laden's determination to obtain a nuclear weapon, or Kim Jong Il's decision on whether to sell him one?

Clearly, the impact is not direct. Indeed, if the United States and Russia eliminated their nuclear weapons, North Korean or Iranian weapons would be more valuable, not less.

Obama will argue that by making a credible down payment on their own commitments, the United States and Russia are better positioned to persuade other nations to strengthen constraints on their nuclear-related activities. The administration hopes that signing New START will impart momentum to next week's nuclear security summit in Washington and the May conference to review the NPT.

The summit is to focus on the most effective way to prevent the most destructive weapons from falling into the deadliest hands. If things go according to plan, the leaders should jointly pledge that before the end of 2012 all nuclear weapons and weapons-usable nuclear material worldwide will be secured to a "gold standard" -- beyond the reach of thieves or terrorists.

That event provides an unprecedented opportunity for Obama to talk candidly with Chinese President Hu Jintao about convincing Kim Jong Il that selling a nuclear weapon to al-Qaeda could have the same consequences as attacking the United States with a nuclear missile. He can also engage the presidents of Pakistan and India about constraining programs on track to significantly expand their arsenals and stockpiles.

President Obama has clearly painted a more hopeful picture of what he calls "nuclear spring." Whether his administration can orchestrate the levers of power as well as persuasion to produce the actions required for success remains to be seen.

Graham Allison is director of the Belfer Center for Science and International Affairs at Harvard's Kennedy School of Government and the author of "Nuclear Terrorism: The Ultimate Preventable Catastrophe."