Leaders of the United States, the Soviet Union and 20 other nations will sign an arms treaty today that painstakingly establishes numerical parity in conventional forces between NATO and the Soviet-led Warsaw Pact.

But Europe is already disarming faster than negotiators can sign treaties.

For nearly two years, military forces throughout Europe have been in retrenchment or outright retreat. Defense budgets are rapidly declining, propelled as much by public demand for action on long-neglected domestic problems as by the relaxation of East-West tensions. The Warsaw Pact itself has practically ceased to exist.

"If the {conventional forces} treaty had been agreed in isolation from current political events in Europe, it would have been a landmark development," said Jonathan Dean, a former U.S. conventional arms negotiator. "But the treaty's achievements have been overshadowed by the dissolution of the communist system in the U.S.S.R., the collapse of the Warsaw Pact and the negotiated withdrawal of all Soviet forces from Central and Eastern Europe -- as well, of course, as by the {Persian} Gulf crisis."

In today's world of scrambled East-West relations, the best troops of the Soviet Army are now deployed on German territory under the North Atlantic Treaty Organization, where they are supported partly by Western funds and protected from hostile citizens by the local police.

Moreover, the United States and its NATO allies last month quietly gave their official blessing to the creation in effect of a nuclear-free zone on the territory of what was East Germany -- an idea widely pilloried in the 1980s as an improbable proposal of the far left.

In these and other ways, Europe's dramatic political changes are producing situations that once seemed unthinkable. Bureaucrats and arms planners, conditioned by the Cold War, are now being forced to reappraise the structures and aims of some of the world's largest standing armies and revise predictions about how far disarmament can go.

The rush of reform has left politicians without a clear blueprint for efforts to control the tens of thousands of advanced weapons that will remain in Europe after the new treaty is implemented. Negotiations are expected in coming months on national troop levels and short-range nuclear forces, but there is no agreement yet on aims.

Overall military trends are nonetheless evident, as illustrated by dramatic changes in Germany, long one of the world's most heavily militarized states and the chief battleground for the U.S.-Soviet war of nerves during the past four decades. Already Germany is gradually being emptied of most foreign troops and weapons of mass destruction.

The Soviet Union, for example, has withdrawn more than 12,000 troops in the past two years, along with hundreds of tanks and associated weaponry. Moscow also pledged several months ago to withdraw the remaining 363,000 Soviet troops by 1994 as part of an overall political retreat from Eastern Europe.

The United States has announced withdrawal of 50,000 troops and several hundred of its best tanks from Europe in coming months for immediate service in Saudi Arabia, as part of the buildup there countering Iraq's invasion of Kuwait. If attempted as recently as two years ago, such a transfer of forces from the old front lines of Europe to the current crisis zone in the Middle East would likely have caused turmoil in every NATO capital; now there is little appreciable public dissent. Some U.S. officials predict the withdrawal of at least another 100,000 personnel from Europe by the end of the decade, which would leave roughly 100,000 American troops in Central Europe.

France, Britain, Belgium, Canada and the Netherlands are discussing withdrawing a total of roughly 90,000 troops from Germany, while the government of Chancellor Helmut Kohl has pledged to cut German forces from roughly 560,000 to less than 370,000 troops by 1992. Further German reductions are expected as neighboring Polish and Czechoslovak armies shrink by more than 30 percent in the mid-1990s.

"East European defense ministers are learning what their counterparts in the West already know: In the absence of a clearly defined and imminent external threat, it is very hard if not impossible to maintain a high level of defense spending," said Dale Herspring, adjunct professor at Georgetown University.

Military budgets have been trimmed and forces moved from the western borders of the five East European nations formerly allied with the Soviet Union in the Warsaw Pact, a fractured group that is to be officially disbanded as a military organization by July. Hungary last week announced it will unilaterally scrap its entire force of Soviet-made tactical missiles.

U.S. nuclear-tipped weapons in Europe, which once totaled more than 6,500, have already declined to fewer than 3,500. Roughly 135 modern U.S. missiles armed with nuclear warheads have been withdrawn from Germany alone under the 1987 U.S.-Soviet Intermediate-Range Nuclear Forces Treaty. An additional 1,200 or so nuclear-tipped artillery shells are scheduled for eventual withdrawal from western Germany.

Moscow has already withdrawn all of its own nuclear armaments from German territory, as part of a July deal between Kohl and Soviet leader Mikhail Gorbachev that also includes a requirement for German payments to support the remaining Soviet troops and bars any future stationing of "carriers" of Western nuclear weapons on former East German territory.

"There are many in {Washington} . . . who believe this is just the harbinger of what's going to come next, meaning that the {nuclear-free} area will grow . . . to encompass not just all of Germany, but a good part of NATO territory," said Catherine M. Kelleher, a University of Maryland expert on German military affairs.

Last week, Germany and the Soviet Union became the first members of the old Eastern and Western political blocs to sign a nonaggression pact, marking what Italian Foreign Minister Gianni De Michelis called the "first sign of closed conflict between the West and the Soviet Union."

The United States two months ago also removed from Germany an arsenal of artillery shells filled with 400 tons of poison gas, dating from the late 1960s. The lethal gas has been shipped to a remote Pacific island for eventual incineration under a U.S.-Soviet agreement to destroy virtually all such weapons.

The new treaty on European conventional weapons due to be signed today codifies and expands many of these cuts by requiring the destruction of roughly 53,000 weapons stationed in the immense region between the Atlantic Ocean and the Soviet Ural Mountains. The Soviet Union and its former allies, which have long had numerically superior forces in Europe, are responsible for destroying roughly 95 percent of this total.

Included are thousands of tanks, artillery, armored combat vehicles, aircraft and helicopters produced at a probable total cost of hundreds of billions of dollars for the war that has never come. By 1995, they must be cut into pieces, exploded, smashed, or -- in the case of combat aircraft -- possibly destroyed in flight as practice target drones. Limited exceptions are allowed for converting some equipment to civilian use and for museum displays.

The treaty forces the Soviets to reduce weaponry in their western region by roughly 40 percent below the level that would otherwise exist after complete withdrawal from Eastern Europe. "The net effect is that NATO, always a qualitatively superior fighting force, will now also have undisputed numerical superiority in Europe," said Lee Feinstein, assistant director for research at the Arms Control Association.

The reductions will be monitored by more than a thousand inspectors from every nation, annually visiting a small fraction of the hundreds of thousands of inspectable "objects" or weapons.

Critics of the new accord note it still leaves in place forces larger than those that existed at the outbreak of World War II. The treaty also allows destruction of the oldest and least effective weapons, without constraining modernization of the remaining forces.

But the next round of negotiations on conventional arms, involving a larger group of 34 nations, is expected to begin in 1992. And if recent history is any guide, public pressures to cut defense spending will likely have as much impact as any future treaty.