Eight months, three presidential meetings, countless Geneva negotiating sessions and one missed deadline later, the United States and Russia appear on the verge of agreeing to a new arms control treaty that will reduce their strategic nuclear arsenals by at least one quarter. But even if the two sides manage to bring home a deal in coming days as they hope, that will be the easy part. After President Barack Obama and President Dmitri A. Medvedev of Russia sign the new pact, they plan to send negotiators back to the table next year to pursue a far more ambitious agreement tackling whole categories of nuclear weapons never before subject to international limits. The talks envisioned for 2010 would take Obama's disarmament agenda to the next level and attempt what no president has managed since the dark days of the Cold War. In addition to further reducing deployed strategic warheads, the negotiations would try to empty at least some vaults storing warheads in reserve. And the two sides would aim at thousands of tactical nuclear bombs most vulnerable to theft or proliferation, some still located in Europe 20 years after the fall of the Berlin Wall. The effort is part of a broader initiative by Obama toward eventual elimination of all nuclear weapons and to transform the U.S. military for a new era. A nuclear review next month will propose an overhaul of the nation's strategic doctrine and force consideration of how many weapons the United States really needs without a superpower rival, including whether to eliminate one leg of the traditional 'triad' of submarines, missiles and bombers. The first step is the completion of the treaty on the table. Obama left Washington on Thursday night to fly to Copenhagen, where he will meet with Medvedev at a global climate change conference. There, they hope to cut through the remaining obstacles to the agreement to replace the Strategic Arms Reduction Treaty of 1991, known as START, which expired Dec. 5. The new version of Start would require each side to reduce deployed strategic nuclear warheads to roughly 1,600, down from 2,200, according to a senior American official. It would also force each side to reduce its strategic bombers and land- and sea-based missiles to below 800, down from the old limit of 1,600. Foreign Minister Sergey V. Lavrov of Russia said Thursday that there had been 'some slowing down' in negotiations by the other side, but U.S. officials denied it and said there were just three remaining issues to resolve, mainly on verification. If lingering differences can be addressed, the Obama administration hopes to plunge right back into talks for a broader agreement. That broader treaty would reduce the number of deployed strategic warheads even further, perhaps to about 1,000 for each country, considered the lowest the two would go without bringing in China, Britain, France and other nuclear powers. Beyond that, negotiators would tackle stored strategic weapons and tactical weapons, neither of which has been limited by treaty. The United States has about 3,000 strategic warheads in storage while Russia has about 1,000, according to the Center for Defense Information, a private advocacy group in Washington. The lopsided balance is the opposite for tactical warheads, generally defined as those with ranges below 300-400 miles. Russia has 3,000-8,000 of them, according to the Center for Defense Information. The Federation of American Scientists estimates that about 2,000 of them are actually deployed, while the Arms Control Association says that perhaps just a few hundred are truly operational. Estimates of U.S. tactical nuclear weapons range from 500-1,200, with about 150-240 still deployed in Belgium, Germany, Italy, the Netherlands and Turkey, half as many as about five years ago. The United States in recent years has withdrawn tactical nuclear weapons from bases in Britain, Germany and Greece. 'Today these weapons are militarily unnecessary, and they are a much bigger liability than asset because Russia and the United States have to maintain security over these warheads whether they are deployed or not deployed, and they're harder to track because they're smaller,' said Daryl G. Kimball, executive director of the Arms Control Association. But the challenge of reaching an agreement would eclipse the difficulties in drafting the current treaty, which was initially supposed to be completed by the time START expired two weeks ago. 'It would make this look like a walk in the park,' Kimball said. The idea of withdrawing all tactical nuclear weapons has generated debate in Europe. In October, Germany's new foreign minister, Guido Westerwelle, called for 'a country free of nuclear weapons,' meaning it was time for the United States to remove the remaining tactical weapons. But other NATO allies are leery of a complete pullback, seeing the presence of the weapons as a sign of America's continued commitment to European security. Similar debates have played out within both the U.S. and Russian military establishments. 'This is what both presidents say they want, but they're both going to have to overcome the resistance of the conservative nuclear bureaucracies in their countries,' said Joseph Cirincione, president of the Ploughshares Fund, a group that advocates disarmament. 'These are small but still powerful forces.' Tactical nuclear weapons were developed during the Cold War as generally lower-yield, shorter-range explosives that could be used on the battlefield. The United States and its NATO allies relied on them as a deterrent to any invasion of Western Europe by what were presumed to be superior Soviet and Warsaw Pact land forces. But since the demise of the Soviet Union, the thinking has flipped, and Russia today views tactical nuclear weapons as a bulwark against American conventional supremacy. 'The idea that they would give these things up lightly is a fool's errand,' said Henry D. Sokolski, executive director of the Nonproliferation Policy Education Center in Washington. 'They're putting more emphasis not less on these systems. They're relying on them more because they can't catch up with us on conventional forces.' Washington and Moscow emerged from the Cold War determined to reduce tactical nuclear weapons, and both sides announced unilateral reductions in 1991. As a result, 17,000 tactical nuclear weapons were withdrawn from service, but no treaty ever imposed legally binding limits. Nikolai N. Sokov, a former Soviet arms control negotiator now with the James Martin Center for Nonproliferation Studies at the Monterey Institute of International Studies in California, called it 'the longest deadlock on the entire arms control agenda,' and he said it might be too hard for the Obama administration to address both tactical and strategic weapons simultaneously. 