A secret offer late last year by West German Chancellor Helmut Kohl to slow down his country's unification was rejected by President Bush, a decision that now produces regrets and concerns here about Germany on the loose.

Kohl's unreported proposal was to sensibly ''confederate'' East and West Germany for a two-year delaying period prior to unification. Bush turned down the chance to give the West time to prepare for the advent of a powerful new Germany.

Saying that he did not want to ''pressure'' Mikhail Gorbachev, Bush told Kohl reunification was the chancellor's issue to be handled as he wished. That opened the door to 50 hours of secret talks between Germany and the Soviets, ending in this week's surprising German-Soviet agreement at Stavropol.

With Gorbachev now pushing for a new treaty-based Soviet-German entente and billions in Western aid, this bit of secret history raises fears of similar administration failure to control what comes next. Such an entente could bypass NATO, reduce U.S. influence on the Continent and give Moscow unprecedented influence in central and Western Europe. Meanwhile, billions for Moscow from the West may end up in a rat hole.

Bush and Secretary of State James A. Baker III are being warned by politically astute figures in Europe and on Capitol Hill to check the stunning pace of events being set by Germany and to tear up the reunification blank check they gave Kohl. That would press Kohl to demand Soviet reform and military cutbacks in return for the financial aid he is giving Gorbachev.

Despite soothing praise from the West's high command for Kohl's big victory in Stavropol, deep unease is the real mood in NATO capitals. This does not convey distrust of Kohl, a valiant soldier for the West during the past decade. Nor is it a yearning for the strategic simplicities of the Cold War.

The unease stems from the unknown: the shadowy potential inherent in the new economic partnership between the Soviets and the Germans. After 40 years of dancing to Washington's tune, Bonn is on the loose and eyeing a new partner.

The president held out against committing U.S. financial aid to Gorbachev at the economic summit in Houston (while not attempting to stop Kohl). But private talk inside the administration leaves no doubt Bush wants to fill the aid cup brim full as soon as the International Monetary Fund finishes the study ordered at the summit and flashes a predictable green light.

Sen. Bill Bradley (N.J.), a strong Democratic voice on post-Cold War U.S. policy toward Moscow, warned that the Soviets have not yet taken the ''difficult steps to reform their economy.'' The way to help the Soviet Union, he told us, is to negotiate ''deep arms cuts.'' Bradley noted Gorbachev's plea to Kohl this week for a sharp slowdown in Soviet debt repayment to the West, already estimated at $3.4 billion in arrears.

While the United States keeps nagging Gorbachev on runaway Soviet arms spending, now nearly 25 percent of gross national product, the record yields no evidence that the administration has even asked Kohl to make the same plea. Yet it is Kohl, not Bush, who could make such pressure effective.

''The economic situation puts Germany in the driver's seat today,'' Sen. Sam Nunn (D-Ga.) told us. As Senate Armed Services chairman, Nunn is trying to prevent evisceration of the new defense budget. That formidable task becomes hopeless if the United States joins Germany in sending cold cash to Moscow without reform of the bankrupt economic system and the end of assembly-line missile production.

British Prime Minister Margaret Thatcher is the wise Western stateswoman who sees trouble ahead for Europe if the United States fails to restrain the jubilant, victorious chancellor, who glimpses electoral glory in the Dec. 2 all-German elections the closer he moves toward Gorbachev.

At dinner two months ago in Cambridge to celebrate the 40th anniversary of the Anglo-German Society, Thatcher listened to a German diplomat speak enthusiastically about impending German unity after 40 years. She replied firmly that in her view reunification was coming ''40 years too soon.'' Now the hour has arrived, two years sooner than it might have had Bush jumped at Kohl's offer last December.