As Congress prepares to draft one of the most pivotal defense budgets in the past two decades, tensions have escalated between the Hill and the Pentagon over reshaping the nation's armed forces in response to the domestic budget deficit and dramatic political changes in Eastern Europe.

Uncertainties over deficit projections have added to the tensions, filling the debate with a confusing array of numbers, budget documents and projections.

"The debate is chaotic," said Gordon Adams, director of the Defense Budget Project, an independent organization that analyzes defense spending. "The budget negotiations are way behind the curve of history."

The Armed Services committees, which in normal years by now would be completing mark-up on the defense authorization bill, have not even scheduled a date to begin those debates because administration and congressional budget leaders have not reached accord on a number for the government's total budget.

This year, as usual, the Defense Department is juggling three budgets: straggling details left over from the 1990 budget, the new 1991 fiscal year budget and the military's long-range plans for the 1992-97 budget.

To complicate matters, fast-changing world events and domestic economic projections have forced Defense Secretary Richard B. Cheney to revise his 1991 budget proposal several times since he sent it to Congress six months ago. He is likely to propose even more amendments after the services complete reviews of some of their most expensive programs, including the new Sea Wolf attack submarine and the Strategic Defense Initiative.

Cheney, a political conservative, has been trying to protect the armed services from erratic cuts at a time when many members of Congress and growing public opinion is demanding larger reductions. But even as he tries to shore up his 1991 requests, Cheney is working on reductions to future budgets stretching through 1997. In addition, Cheney has unhappily watched the defense budget become the bank for financing other programs, most recently aid to Panama and Nicaragua.

"He has an unusually tough job dealing with the total picture," said Rep. William L. Dickinson (Ala.), ranking Republican on the House Armed Services Committee. "He has less support coming from the White House than his predecessors have . . . . Cheney is rolled fairly often on things he's requested. That's a big change from {the} Reagan {administration years}."

Many lawmakers claim that tensions have been exacerbated by an information void from the Defense Department as Cheney attempts to develop those long-term budgets.

The numbers under consideration for the 1991 fiscal year include:An administration request for $306.9 billion in spending authority that it says would mean troop cuts of about 38,000, including two Army divisions. House approval of a budget resolution that would allot $283 billion to defense, about $23.9 billion less than the administration wants. Cheney has said those figures will translate into cuts of about 140,000 troops. A Senate Budget Committee vote to cut President Bush's defense spending authority to $285.6 billion -- $21.4 billion less than the request.

"The only place we can get those kinds of savings in 1991 is manpower," Cheney told the Senate Appropriations subcommittee on defense last week. "There's no other place to go. We'd have to run a lot of people off, throw people in the street, freeze promotions."

Cheney's personnel chief, Christopher Jehn, said in an interview last week that the military cannot reduce more than 50,000 to 75,000 troops a year without large-scale involuntary separations.

About 47 percent of the defense budget is swallowed by personnel costs, and most of that is spent within the year it is authorized, according to Cheney.

Another 38 percent of the budget is used to pay obligations from previous years, such as the development and production of weapons, with the remaining funds in the 1991 budget used to pay for weapons and other costs during that spending year.

But the 1991 budget is only part of the debate. Members of Congress complain they are hard pressed to make decisions on the 1991 budget because they lack details of the Pentagon's long-range plans. Pentagon officials say they are still shaping long-term strategies to be presented to the Hill in January.

Even so, Cheney has yet another set of numbers: On Tuesday he is scheduled to deliver figures to Congress showing the impact of a 25 percent force structure cut for the 1991-1995 fiscal years. He hinted in a committee hearing last week this could mean cutting five to six Army divisions and five to six Air Force wings.

Those numbers are similar to yet another set of figures -- the armed services' long-range spending projections for 1992-97, the Program Objective Memorandas, which are now under review by Cheney and his staff.

In those documents, the services have proposed cutting 310,000 uniformed service members by 1997, including six Army divisions and eight Air Force tactical fighter wings.

The projection of such future deep cuts has made many members of Congress question why Cheney is so adamantly opposed to what many legislators consider justifiably sharp reductions in the 1991 budget. Many lawmakers, including House Armed Services Committee Chairman Les Aspin (D-Wis.) complain that Cheney's proposed budget for the year beginning Oct. 1 "is based on threat assumptions that are outdated," and does not recognize improved superpower relations.

Cheney, who represented Wyoming in the House for a decade before becoming Bush's defense chief, has frequently derided members of Congress for advocating deep military cuts while balking at base closings and fighting many of his efforts to kill weapons programs.

Congressional committee staffers now preparing for mark-up on the administration's proposed $306.9 billion defense authorization bill are more pessimistic in their outlook. "It's going to be a blood bath," said one senior aide to the House Armed Services Committee.