Rarely are there moments like the vote that will take place in the Senate on October 12. A Senate rejection of the Comprehensive Test Ban Treaty (CTBT) would scuttle far more than one arms control agreement. It would irreparably damage global efforts to contain the spread of nuclear weapons; undermine, if not end, American leadership of these efforts; and risk the collapse of the treaty regime painstakingly constructed over 50 years by Republican and Democratic leaders.

The impact will not be felt immediately. A Senate vote in favor of nuclear weapons tests, however, will almost certainly trigger a chain reaction deeply harmful to U.S. national security interests.

A Return to Global Nuclear Tests.
If the United States does not ratify the test ban treaty, Russia will certainly not ratify the agreement, nor will China. Without China’s ratification, India will not sign the treaty and without India, Pakistan will not. Over the next two years it is highly probable that one or all of these nations will resume testing of nuclear weapons, whether or not the United States continues its moratorium on testing.

The Indian government is already under pressure from its military and scientific communities to test again. The collapse of the international norm against testing will remove any reason for the government to resist these demands. Pakistan will likely follow suit. China, decades behind the U.S. in nuclear testing expertise, would want to quickly resume the nuclear test program cut short by the completion of the CTBT in 1996. China will also view expanded U.S. military cooperation in East Asia, including missile defense deployments with Japan, South Korea or Taiwan, as a threat to its nuclear arsenal and wish to develop a new generation of lighter, more compact nuclear weapons—possibly influenced in part by U.S. technology gained through espionage.

Russia is relying more on its nuclear weapons to compensate for conventional military weakness. Deputy Minister of Atomic Energy Viktor Mikhailov recently complained that the large size of nuclear weapons made "a real nuclear war" impossible and said, "Nuclear pressure will again become an effective political instrument if the threat of nuclear strikes is made more real. For that, it is necessary to have the possibility to inflict ‘pinpoint,’ low-yield nuclear strikes on military targets located anywhere on the globe." Russia interest in new tests of such weapons will increase if the Senate rejects the CTBT.

Increased Regional Instability.
For the states listed above, the CTBT is the only international restraint to resumed testing. There are 182 non-nuclear-weapon states party to the nuclear Non-Proliferation Treaty (NPT) who have sworn not to develop or acquire nuclear weapons. In a world where reliance on nuclear weapons is increasing and where nations are openly testing new designs and new missile systems, several nations are certain to recalculate whether continued adherence to the NPT improves their national security. With no international norm against nuclear testing, clandestine efforts in Iran, North Korea and Iraq may break out into the open and even cross the test threshold.

Japan could well feel heightened pressure to reevaluate its adherence to the NPT. It has ample ability to construct modern nuclear weapons should it so desire. Faced with a weakened international regime, uncertain U.S. adherence to international commitments and the emergence of new nuclear nations, Japanese leaders may believe that they have no choice but to develop their own nuclear deterrent, fundamentally altering the global strategic landscape.

Global Nuclear Insecurity.
The CTBT is not just another treaty. It is one of the lynchpins of the non-proliferation regime, the interlocking network of treaties and arrangements assembled under U.S leadership. The pledge by the nuclear-weapon states to end nuclear testing has long been considered the minimum necessary fulfillment of their obligations under the NPT to reduce and eventually eliminate their nuclear weapons. The Limited Test Ban of 1963 was to be only a rung on the ascent to a total ban. The NPT explicitly calls for the test ban as does the "Principles and Objectives for Non-Proliferation" agreed to in 1995 when the nations of the world agreed to make the NPT a permanent treaty.

Without the test ban, many nations will conclude—even without the explicit regional developments outlined above—that the NPT is a hollow treaty. Fearing that their neighbors might break out of the treaty restraints, they will begin to hedge their bets. The immediate casualties will include efforts by the United States to conclude a treaty ending the production of fissile material (the plutonium and highly-enriched uranium needed for bombs). The United States and Russia are awash in the material, but other nations will not wish to limit their current or potential supplies in an uncertain nuclear environment. Negotiations for further reductions in nuclear arsenals, already in trouble, will be even more difficult to revive. With large nuclear arsenals in several nations, the possibility of theft, sale or sabotage will increase.

Weakened U.S. Leadership.
The damage from a failed test ban treaty would extend beyond the non-proliferation arena. Key U.S. allies will feel betrayed, some outraged. Fifteen of the nineteen NATO nations have already ratified the treaty, including the only European nuclear-weapon states, France and the United Kingdom. NATO leaders are unanimous in their support for a test ban. After years of U.S. efforts by both Republican and Democratic administrations to secure the treaty, a Senate rejection at this late date will seriously undermine U.S. credibility. For nations already concerned about U.S. unilateralism, a rejection will be taken as further evidence that the U.S. cannot be trusted to honor its commitments or consider the counsel of even its closest allies.

This may be the most profound vote many Senators will ever cast. The time is short; considered debate is limited. The stakes couldn’t be higher.

Joseph Cirincione is director of the Non-Proliferation Project.