After nearly a decade of negotiations, the United States and the Soviet Union don't seem able to finish a treaty that would reduce their arsenals of long-range nuclear missiles, submarines and bombers.

But if Washington and Moscow don't complete a treaty soon, the opportunity for nuclear-arms control may be lost for years to come.The conventional wisdom is that a strategic arms treaty is a Cold War relic and thus no longer a U.S. foreign policy priority.

Some say arms control is unnecessary because the Soviet Union's deteriorating economy already establishes a sufficient check on its future military growth.

The conventional wisdom is wrong. It assumes that the Kremlin is neither willing nor able to continue its aggressive nuclear modernization.

Given the disturbing changes in the Soviet Union, which has 30,000 nuclear weapons, that could turn out to be a dangerous illusion.

START could play a vital role in shoring up nuclear stability in an unpredictable moment in Soviet history.

The nearly completed accord would do three important things.

First, it would accomplish the core U.S. goal, which is to focus any nuclear reductions in areas of Soviet military advantage. Thus, the Soviets would be obliged to cut ballistic-missile warheads by more than 40 percent and their highly destabilizing SS-18 missiles by 50 percent.

Second, the treaty would protect advanced military technologies, which would be vital for protecting U.S. interests in future conflicts. In particular, it would not constrain the deployment of highly flexible conventional cruise missiles.

Third, the accord would establish a regime for intrusive monitoring and verification, including a dozen forms of on-site inspection.

These aspects of the treaty have not gone unnoticed by the Soviet generals and the military-industrial complex.

With the balance of power shifting in favor of the military hard-liners, the hard-fought U.S. victories in START are in jeopardy.

For example, there is a growing debate in the Soviet military about the vigorous inspection system, with critics charging that it would enable U.S. inspectors to engage in "legalized espionage."

For the Soviet military, START does not sufficiently limit new high-tech systems such as superaccurate U.S. conventional cruise missiles.

More importantly, the Soviet high command could decide to place new emphasis on its own nuclear arsenal in a strategy designed to offset U.S. technological superiority in conventional arms.

Such a development could not come at a worse time.

With the military wielding growing influence in internal politics, it could mean Mikhael Gorbachev or any other Soviet leader would have little choice but to accede to a military push for scuttling START and preserving and modernizing the huge Soviet nuclear arsenal.

We probably still have a "window" for completing the accord, especially now that differences on carrying out last year's accord on conventional forces in Europe seem to have been resolved this week.

Gorbachev still wants a nuclear agreement.

But a delay in finishing the accord would mean that a large portion of the existing treaty would have to be renegotiated under difficult circumstances.

At worst, a delay could mean no treaty at all, together with a more militarized - and nuclearized - Soviet foreign policy.

We should not permit the Soviet generals to get their way. We should finish START now.