THERE IS no longer any real doubt that the government of Pakistan is building (and will shortly have) a nuclear weapon. In response, Congress is reviving threats to cut off all military aid to Pakistan unless Islamabad promises to cease its nuclear activities. This threat is almost certain to be ineffective because it fails to address the real problem: how to defuse an impending nuclear race on the Indian sub-continent.

Pakistan's security concerns are focused principally on India's potential nuclear capability. It has always been unlikely that sales of F16s and other conventional arms would, by themselves, remove that concern. Instead, Pakistan President Zia-ul-Haq has shrewdly gambled that, given his country's value as the only conduit for U.S. arms to the Afghan resistance, he could have both his nuclear weapon and an American-supplied and modernized conventional military to boot. Nor is he likely to be moved by a renewal of congressional threats.(Recall his now famous "peanuts" response to President Carter's offer to resume aid after cutting it off in 1979. The "peanuts" were several hundred million dollars of military aid.)

Why should we expect Zia to stop the bomb when he has within reach a prize that would enhance his prestige in the Islamic world, and also be a credible counter to India's simmering weapons program? It is doubtful that Zia would regard short-term U.S. conventional military aid as more valuable in the long-run than an indigenous military nuclear capability -- especially since a Soviet withdrawal from Afghanistan would sharply reduce pressure for U.S. aid anyway. Zia also knows how forgiving the American Congress can be when it is moved by the vague but perennially attractive hope of retaining at least one "reliable" American ally in Southwest Asia.

Threats of aid cutoffs might havebeen credible during the bomb program's embryonic stage in the early 1970's, before the invasion of Afghanistan made Pakistan's strategic value apparent. But severing U.S. aid now would be foolish simply because of the reality: A nuclear -- armed Pakistan is now inevitable.

President Reagan can no longer claim with any credibility that Pakistan is not developing nuclear explosives. The uncovering of an extensive international network created to smuggle illegal bomb materials into Pakistan has removed any reasonable doubt. The recent arrest of a Pakistani-born Canadian caught trying to smuggle into Pakistan an alloy crucial to nuclear weapons production is only the latest (albeit for the White House, the most embarrassing) example of Zia's determination to achieve covertly what he could not afford to pursue openly.

It is time for the president to state publicly that the United States can no longer hope to enforce its non-proliferation policy solely by denying technology and financial support to would-be nuclear powers. We are entering a new and frightening era in which strategies must be crafted to avoid a metastatic spread of small nuclear arsenals. With the decline of unilateral leverage, the United States must work to create new multilateral frameworks that move beyond the assumptions that created and continue to motivate the 1968 Nuclear Non-Proliferation Treaty (NPT) and the safeguards established by the International Atomic Energy Agency (IAEA).

Any realistic strategy must bring both Pakistan and India into the community of nuclear weapons states on the logical assumption that Zia and Indian President Rajiv Ghandi will be less dangerous to each other, and to the rest of the world, within the nuclear fraternity than outside of it. Although Pakistan cannot be turned back by any method short of a strike against its nuclear facilities, it is less clear that other "threshold" states -- for example, South Africa, Argentina, Brazil -- are beyond the point of no return. Israel, while not technically a "threshold" state (since by now it certainly possesses a nuclear arsenal of its own), should also be brought into the new regime. Allowing Israel's nuclear program to continue without official protest casts an embarrassing shadow over U.S. condemnations of Pakistan.

The United States should propose a new treaty, the Nuclear Threshold Agreement (NTA), including the current "nuclear threshold states" in a binding arrangement designed to keep the size of new nuclear arsenals within agreed limits. Signatories would: (1) foreswear first use of nuclear weapons; (2) agree to a multilateral inspection regime monitored by the IAEA; (3) pledge not to provide nuclear help to other states; (4) contribute to a confidential data base to include a comprehensive inventory of weapons, amount of fissionable (i.e. bomb) material, reprocessed reactor fuel, etc. These measures should reduce, for example, the Indian government's incentive to launch a preemptive strike against Pakistan's nuclear installations, an option that (according to private accounts) has gained support over the last several months.

Admittedly, it is unclear whether this new regime would significantly retard progress toward a weapons capability by threshold states. But creation of a new control mechanism, specifically tailored to threshold nations is likely to be more effective than simply adding new clauses to the NPT -- which has been rejected by the threshold nations on the grounds that it imposes restrictions and safeguards on them that do not apply to signatories already having nuclear weapons.

The NPT, moreover, was never designed to detect diversion of nuclear material for clandestine bomb programs -- which is why it has been so ineffective as a proliferation retardant. Nor can the current regime, which relies heavily on detection of testing, deal with the fact that threshold nations may be able to build workable nuclear devices without testing. And a new treaty would certainly provide more reliable data on the status of nuclear programs, data that would be valuable should any of these countries become embroiled in a conventional conflict where nuclear escalation is a possibility.

The failure to have sounded the alarm about a Pakistani bomb when we could have stopped it shows the need for prudent, farsighted planning to prevent future nuclear renegades. Instead of focusing on how to punish Zia, we should seek to benefit from the mistaken policies that allowed him to get this far.

Jed Snyder was senior special assistant to the director of politico-military affairs at the State Department during 1981-82. He is deputy director of national security studies at the Hudson Institute's Washington Office.