FROM its inception, the Strategic Defense Initiative (SDI) has been on a collision course with the Anti-Ballistic Missile (ABM) Treaty. This conflict has come to a head with the Senate's approval of a proposal by Armed Services Committee chairman Sam Nunn to amend the ABM Treaty to accommodate SDI. The Senate has it backward: It should be trying to reform SDI to preserve the ABM Treaty.Signed in 1972, the ABM Treaty was the first and most lasting achievement in strategic arms control. It bars the United States and the Soviet Union from deploying nationwide defenses against long-range missiles - strategic defenses. The superpowers recognized that defenses would only create incentives for larger, more capable nuclear forces. The ABM Treaty has allowed the superpowers to restrain the nuclear arms race and build a stable relationship; it remains a key condition for future reductions in their vast nuclear arsenals, bloated relics from the cold war. Senator Nunn's amendments would cut at the heart of the ABM Treaty by permitting the nationwide deployment of strategic defenses. The best way to promote nuclear arms reductions would be to protect the ABM Treaty by incorporating its constraints into the structure of the SDI program. Specifically, the US should divide SDI into three sections with three separate missions, each subject to different treaty constraints. This would not stand in the way of SDI's most widely accepted aims, but would rein in its more dubious elements. * SDI's first mission should be to develop and deploy defenses against short-range missiles, so-called tactical missile defenses. Missiles with ranges of hundreds of miles are now widely proliferated in the Middle East and elsewhere, where they can be used as terror weapons against neighboring countries. To enforce its limits on strategic defenses, the ABM Treaty requires that tactical defenses not be given the capability to intercept long-range missiles. The treaty does not specify how to measure this capability, but many reasonable definitions have been proposed, and the US and Soviet Union should be able to agree to one. One proposed tactical defense system, the Theater High-Altitude Area Defense (THAAD), appears to exceed any reasonable threshold for strategic ABM capability. * The second SDI mission would be to develop and test ground-based strategic defenses. Limited defenses have been proposed to defend against the accidental launch of a few Soviet missiles, or potential long-range missile threats from the third world, but these are both relatively remote dangers. For now, the drawbacks of deploying these systems outweigh their claimed benefits. The Soviet Union has made it clear that US actions that undermine the ABM Treaty would threaten the future of arms control, including the recently signed Strategic Arms Reduction Treaty (START). Still, the US should continue to develop and test these systems, as allowed by the ABM Treaty, so that it will be prepared to deploy them in case a real threat arises. * The final SDI mission would be to explore technologies for more effective missile defenses in the future. The ABM Treaty prohibits the testing of space-based missile defenses, but current space-based defense concepts like "brilliant pebbles" do not show great promise in any case. Other potential defense technologies, like high-powered lasers, are too immature to warrant full-scale testing. Still, the US should continue research on these technologies; they offer the best hope, however remote, for effect ive defenses against long-range missiles. This restructuring will permit SDI and the ABM Treaty to coexist, reducing the friction that erodes support for both.