Presidents Bush and Mikhail Gorbachev yesterday began struggling with a major challenge now emerging in post-Cold War Europe: how to constrain a united Germany's military capability without seeming to punish it for its role as an aggressor in World War II.

U.S. and allied diplomatic officials say that the issue does not concern the size of future German forces as much as it involves the formula for their reduction and how the agreement is presented, according to U.S., Soviet and other diplomatic officials.

One aspect of the struggle, the official said, is whether the German forces should be limited under a new Conventional Forces in Europe (CFE) agreement between 23 nations or under an arrangement developed in special negotiations involving just the four victorious World War II powers and the governments of East and West Germany.

Another question is how rapidly the limitations should be imposed: in a near-term conventional arms accord known as CFE I, a follow-on accord called CFE II, or something in between, which some call CFE IB. But whatever course is chosen, there is broad agreement that the result will likely be the same.

Both sides recognize that the West German army, or Bundeswehr, will soon shrink. The Bonn government has pledged a 15 percent reduction, from 470,000 to 400,000 active troops, and Defense Minister Gerhard Stoltenberg said in a carefully timed statement this week that additional reductions are expected under a new arms agreement that also limits the forces of Germany's neighbors.

"The issue of the size of the Bundeswehr is of obvious concern to the Soviets," White House spokesman Marlin Fitzwater said in explaining why Bush would confront this issue during his sessions with Gorbachev. The U.S. and allied hope is that resolution of the issue also can be the linchpin to Gorbachev's acceptance of a united Germany's membership in the Western military alliance.

Officials say the challenge that both sides face is to develop limitations that are sufficiently plain to satisfy Gorbachev's conservative military constituency at home but are also sufficiently vague to not offend the domestic constituency of West German Chancellor Helmut Kohl, a strong U.S. ally.

"The Germans are paranoid about singularity and about being set out somehow" as different because of their wartime record, a senior U.S. official said. "That's why, you know, you don't talk about limits on the Bundeswehr, you talk about a CFE in which everybody would be limited . . . {including} the Germans."

German Foreign Minister Hans Dietrich Genscher told Bush and Secretary of State James A. Baker III last week that Bonn was willing to accept special restrictions on armed forces in a so-called central European region that encompasses Germany and several other nations, according to U.S. and allied diplomatic officials.

They said that the formulation of this proposal has not been agreed within the Western alliance, much less widely circulated within the East, a circumstance that may help explain why Bush and Gorbachev said late yesterday they will not settle the German issue during their bilateral talks this week.

Officials have said the Western allies still have not settled whether the Western region subject to special troop limitations should include only Germany, Belgium, the Netherlands and Luxembourg, or whether it should also include France, as Bonn prefers.

"It would be nicer if the region was big, rather than small," a German official said.

But U.S. and diplomatic officials also cautioned that the idea was worth pursuing only if the Soviets gave up any hope of limiting German forces within the so-called Two Plus Four Talks, involving the World War II powers and the two Germanys. A senior U.S. official said last night that the Soviets were "moving in this direction."

"We understand that Bush will try to estimate this week how much room Gorbachev has to maneuver further," a senior official said.

Once the two sides agree on the forum for limitations, the next issue to be confronted is when the constraints must be imposed. A senior U.S. official said Bush's position "going into" the current summit is that the prospect of such limitations can be mentioned in CFE I but should be negotiated in CFE II. Otherwise, he said, the current talks will be needlessly complicated and probably delayed past an avowed deadline of Dec. 31.

But several U.S. officials said the administration was not unalterably opposed to considering earlier restrictions, and German officials have said they also are not opposed if an agreement can be struck quickly. Such a shift would go a long way toward meeting Soviet desires for immediate assurances.

French officials, however, have said that compliance with additional troop constraints may be unverifiable and that the issue should not be allowed to complicate the current negotiations on CFE I. "Our concern in France is not to limit the Bundeswehr; it is to limit the Soviet forces that remain in Europe," one official said.