When the last word is written on the Reagan years, one major puzzle will be the president's headlong leap from frying pan to fire on arms control. The week of Aug. 24 featured two milestones on the road from standoffishness to rashness.

First, the administration suddenly abandoned its longstanding insistence on on-site inspection in the pending treaty on medium-range nuclear weapons. Then came Chancellor Helmut Kohl's surprising concession that West Germany won't modernize its Pershing 1A missiles when they reach obsolescence in the early 1990s. He did so, he said, after ''intensive consultations with our American friends,'' consultations whose tenor isn't difficult to imagine.

For once, the administration's right-wing critics are on the mark. The administration is hellbent on signing an arms-control treaty before Ronald Reagan leaves office. Almost any treaty, however rash or defective, will do.

The subtle pressure on the West Germans is a sign of the new mood. The western summer White House vehemently denied that anyone had pushed the West Germans to meet the latest Soviet conditions. But no one had to push. Kohl's government had been left by us twisting in the wind of domestic and international pacifist opinion -- labeled as the guys who'd cling to a Tinkertoy missile system at the expense of the European nuclear peace.

The Reagan administration's no-concessions-barred policy began to emerge last fall at the sudden Reykjavik summit, a scatterbrained exercise in bidding between Reagan and Mikhail Gorbachev that soon collapsed of its own absurdity.

Unfortunately, no such collapse is in prospect for the far riskier gamble inherent in the pending ''intermediate-range force'' treaty talks, now nearing fruition in Geneva. The United States and the Soviet Union are hurtling full tilt toward the so-called double zero option -- the mutual removal of all theater-range nuclear missiles (between 300- and 3,000-mile ranges) in Europe.

If signed, such a treaty will make a lot of people feel more secure. That they will actually be more secure is most doubtful. The military value (and threat) of these weapons systems is, barring accident, negligible. Their political value is incalculable.

The United States deployed the most recent of these missiles in Europe in the early 1980s, after a hard battle. The Soviet Union had begun and continued a mindless deployment of SS-20 missiles targeted on NATO countries. The pressure for a U.S. response was heavy. Without such a response, the feeling was, Europeans might well ask whether we were in effect ''decoupling'' our fate from theirs.

Having prevailed on the NATO allies to allow deployment of these weapons -- usually in politically costly struggles with their leftist and pacifist oppositions -- we are now on the verge of jerking them out, not some but all. Mere reductions aren't sufficient for the utopians of the Reagan administration. ''Zero'' is the big shibboleth of the moment.

It might seem, superficially, that if we withdrew the new cruise and Pershing-2 missiles, and the Russians likewise dismantled the SS-20s, we would revert to the status quo that existed before the ''Euromissile'' race. Unfortunately, strategic developments of this sort aren't easily reversible, for they implicate and bring into play new psychological factors that affect the credibility of deterrence.

Now as ever, the Warsaw Pact has NATO vastly overpowered, if not outclassed, in conventional weapons and manpower. That mattered less when the United States enjoyed unquestioned strategic superiority (in weapons of intercontinental range), as it did well into the 1960s. Today, it arguably matters more than at any time since NATO came into existence in 1949 to redress the heavy Soviet conventional advantage.

On the day the intermediate-range systems are removed, the military balance on the continent will tip decisively in the Soviets' favor. That is implicit in the logic of the situation. And that is why many of our European friends view the current pell-mell rush to agreement on the INF treaty as a sign of politics and impetuosity.

Gen. Bernard Rogers, the retiring NATO commander, has complained loudly of the rashness of the zero-zero idea, if it is unaccompanied by any adjustment in conventional or intercontinental forces. Rogers is a calm, thoughtful and universally respected officer. He is much closer to thoughtful NATO opinion than the new accommodationists of the Reagan administration.

Zero-zero will render the Soviet Union stronger militarily in Europe than it has been for a generation. And this state of affairs will have been ushered in by the president who spent most of his first term denouncing Soviet deceitfulness. The reversal would be comic if it weren't so very dangerous.