The defeat of the Comprehensive Test Ban Treaty (CTBT) on Oct. 13 by a handful of hard-line Senate Republicans was a blow to more than 40 years of U.S. leadership on nuclear nonproliferation. It has left friend and foe alike scratching their heads in confusion at how the world's greatest deliberative body wandered into such an untenable mess. The answer lies in a series of miscalculations and missteps taken by all sides.

Senate Democrats rightly pushed for hearings on the CTBT - and a few pushed for a vote -never expecting the majority leader to allow only one week for hearings before the ratification vote. The Republican leadership -realizing it had set a train wreck in motion - tried to postpone the damaging vote, but failed, because it could not control its own caucus. The White House effort to salvage the treaty was simply too little, too late. The administration should have worked harder to build support for the treaty during the two years it languished in the Foreign Relations Committee.

The foreign policy freefall was just beginning: five days after the CTBT vote, President Clinton vetoed the foreign assistance bill, and a week later, he vetoed the bill containing funds for the State Department's operating budget and U.S. contributions to international organizations. Both bills underfunded major U.S. international commitments, including support for the Wye River agreement and payment of our U.N. dues. But the real problem is not FY 2000 funding levels. The real problem is the 15-year downward spiral in the international affairs budget.

Funding to support U.S. foreign policy has dropped almost 40 percent in real dollar terms since the high-water mark in 1985. Since the beginning of the Clinton administration, the steepest drop in funding for U.S. foreign policy occurred in 1995, with the first budget passed by the Republican Congress, among whose freshman members were some who showed their distaste for foreign affairs by bragging that they didn't even own passports.

In 1997, Secretary of State Albright launched a campaign to drive home the importance of international affairs spending to the life of the average American. Her tenacity and leadership convinced many members of Congress that U.S. foreign affairs spending had been allowed to drop too far, and won support for modest increases in foreign affairs spending for fiscal years 1998 and 1999. Now, even that progress has been reversed.

Support for U.S. engagement in international affairs is far more important than the events of the past few weeks suggest. In an international environment characterized by interdependent economies, porous borders, and non-state actors with global reach, the United States can not afford to abandon its leadership role in the world. Even if we preferred to isolate ourselves -and most Americans do not -globalization has made isolation impossible. The United States cannot be prosperous and secure in an unstable world, and maintaining stability requires U.S. leadership.

And leadership requires that we back our words with action. The United States is the leading advocate for constraints on nuclear-related exports and restraint on nascent nuclear programs. We led the fight for the indefinite extension of the Nuclear Nonproliferation Treaty (NPT) in 1995 and for the completion of the CTBT in 1996. And yet, after only one week of hearings, the Senate rejected that treaty, which many around the world regard as the linchpin of the international nonproliferation regime. That one vote pulled the rug out from under U.S. leadership and credibility on nuclear nonproliferation, which is vital to ensuring our national security in the years ahead.

The United States cannot meet its obligations as a world leader by conducting foreign policy at cut-rate prices - or welching on our debts altogether. When we fail to pay our U.N. dues, we undermine support from our friends and allies who stand by us in places such as Kosovo and Iraq. When we fail to support the State Department budget, our diplomatic efforts suffer, because U.S. embassies are understaffed, ill-equipped and vulnerable to terrorist attack. When we fail to fund our foreign assistance programs, we lose the chance to build a world that more closely reflects American values and interests. U.S. foreign aid supports peace in Bosnia, Northern Ireland and the Middle East. It supports preventative health care, clean drinking water, education, micro-enterprise, and the rule of law. It is preventing the spread of nuclear know-how from the former Soviet Union and is combating the spread of AIDs in Asia. All that for less than 1 percent of the federal budget.

This morning, the president and the Congress are locked in a battle over the budget to see whose spending priorities will prevail. To ensure continued peace and prosperity for all Americans, adequate funding for U.S. international affairs must be a part of any budget agreement. Otherwise, U.S. leadership will continue to erode, and that is something none of us can afford.

Sen. John F. Kerry is a Massachusetts Democrat.