As the United States and the Soviet Union sit down this week to start a new round of talks aimed at reducing conventional military forces in Europe, there is both opportunity and danger in what is being proposed.

The opportunity arises from offers that would dramatically cut the numbers of troops, tanks and aircraft on both sides. The danger is that Europe would be left nearly defenseless.The talks involve the representatives of the 16-nation NATO alliance and the Soviet-led, seven-nation Warsaw Pact. But the United States must pay careful attention to what its allies think, while the Soviets have no such constraints.

And therein lies the problem: The Soviets could drive a wedge into NATO that might destroy its effectiveness, merely with words.

After the U.S.-Soviet deal last year that eliminated all medium-range missiles from the East-West arsenals, the Warsaw Pact nations were left with a rough 2-1 superiority in conventional weapons, including thousands more tanks that could spearhead an invasion.

Most experts felt that the loss of the nuclear weapons would have to be made up by bolstering NATO's conventional forces to counter the numerical superiority of the Soviets.

The NATO countries entered this week's talks asking that the East Bloc nations reduce their huge armored forces by 25,000 tanks - about 50 percent. Only then could both sides start negotiating equal cuts in their remaining forces.

At one time, that would have been unlikely. Yet much has changed under Soviet leader Mikhail Gorbachev, who last fall announced a unilateral withdrawal of six tank divisions - 5,000 tanks - from Eastern Europe.

At the opening of this week's parlay, Soviet Foreign Minister Eduard A. Shevardnadze dropped a radical set of new proposals on the table.

First, both sides would eliminate imbalances in their forces, then further reduce troops and all weapons by 10-15 percent over two or three years. Combat aircraft - the one area where the West holds an advantage in quality - would be included in the cut. In addition, all short-range nuclear weapons would be removed in certain zones.

Second, each side would reduce its forces by 500,000 troops over a two-or three-year period.

Third, the armed forces of each side would become strictly defensive.

All of that goes much further than NATO had planned. And it exploits serious differences of opinion among NATO members about short-range nuclear weapons and the need for more conventional forces.

Gorbachev's apparent willingness to disarm has led many West Europeans to question the need for the NATO alliance at all. The Soviet leader personally is one of the more popular figures in Europe.

Such reductions sound wonderful, but an essentially disarmed Europe is a cause for worry, especially since the Soviet Union is so near and the United States so far away. The thousands of tanks pulled back from Eastern Europe will still be available and those huge Soviet tank factories are still working.

In any event, Gorbachev may be sincere, but he won't last forever. Who knows what might follow?

The arms reduction talks will have to be taken very slowly, with long pauses between steps. America certainly must try to take advantage of the apparent Soviet willingness to deal - but not rush into something that could destroy NATO without a shot being fired.