The first comprehensive attempt in more than a quarter-century to reform the way the Defense Department spends money and prepares for war gets under way this week, led by a bipartisan juggernaut on Capitol Hill.

Propelled by political, economic and diplomatic pressures, the principal reforms will be unveiled Wednesday by two traditional Pentagon allies -- Sen. Barry Goldwater (R-Ariz.), chairman of the Senate Armed Services Committee, and Sen. Sam Nunn (D-Ga.), the committee's senior Democrat. In a thick report that has been two years in preparation, the senators will propose overhauling the way weapons are bought and streamlining the allegedly cumbersome military chain of command.

"There is need for major change," Nunn said.

"The system is broke and it must be fixed," Goldwater added. "The reorganization of the Department of Defense may be the most important thing that Congress does in my lifetime. It will be the most important thing I do in mine. The last time we really did anything significant was almost 30 years ago in the Defense Reorganization Act of 1958."

Goldwater said the study and recommendations will be assessed through committee hearings for the rest of this year and form the basis for legislation next year on how to fix the Pentagon.

The Nunn-Goldwater report dovetails in many ways with other reform movements, in Congress and in the executive branch, that are searching for ways to make the Pentagon more efficient, particularly in the face of a $200 billion federal deficit and public consternation at defense procurement "horror" stories.

In the House, Rep. Les Aspin (D-Wis.), chairman of the Armed Services Committee, has launched hearings to determine whether the $1 trillion President Reagan spent in his first term was used wisely. Aspin has already heard testimony from pro-defense witnesses that the money has not made U.S. conventional military forces much stronger compared to the Soviets.

In an office across the street from the White House, a commission appointed by Reagan to find ways to get more bang for the taxpayers' buck also has found the Defense Department wanting in many areas. Commission chairman David Packard, a former deputy defense secretary, has served notice that he favors overhauling the western world's biggest corporation, the Pentagon.

Within the Pentagon, the reformists have pushed some leaders into a defensive crouch. Defense Secretary Caspar W. Weinberger has disputed the premise that the Pentagon needs overhauling. Navy Secretary John F. Lehman Jr. tried to give Weinberger's theme political resonance by stating that the bloodless forcing down of the Egyptian 737 airliner carrying four pirates last week demonstrated that the armed services are battle ready and highly competent.

Amid this verbal combat, however, admirals and generals are throwing away their "wish lists" for new weapons and more troops. They have realized that, barring war, the days of their budgets growing by an average of 7 percent a year on top of inflation are over. They are cutting back to fit their programs within budgets growing no more than 3 percent for the next five years, if that much.

"We've already cut $90 billion out of our five-year plan," said Gen. John A. Wickham Jr., the Army chief of staff.

That $90 billion would cut an earlier plan covering fiscal 1986 through 1990. Under Wickham's new budget, the Army assumes it will receive increases of inflation plus 3 percent growth in each of those five years, an optimistic assumption in the eyes of many lawmakers.

Wickham's approach to the coming budget squeeze is being duplicated by other military leaders. They are trying to avoid cutting manpower and save money by trimming or scrubbing small programs, while stretching out the big, highly prized ones.

The Army, for example, plans to buy 120 fewer M1 Abrams tanks a year -- cutting back from the planned 840 to 720. The service also will reduce its annual purchase of AH64 helicopters from 144 to 120 and stretch out the Stinger antiaircraft missile program by buying 4,200 a year rather than 8,500.

But Goldwater and Nunn say such economies amount to Band-aids for a hemorrhage. They are recommending fundamental reforms in the way the military runs its business and commands its troops, tanks, ships and planes.

Their report this week will document alleged military shortcomings. Among the assertions:Interservice responsibilities and rivalries have fractured the Joint Chiefs of Staff and theater commanders. An admiral responsible for fighting a war in the Pacific, Nunn complained, is prohibited under the current chain of command from telling Army officers serving under him where to store ammunition. The military's chain of command, in which orders travel from Washington through layer after layer of commands on the way to the field, is dangerously cumbersome and must be streamlined before it costs lives, or even loses a war. Congress and the defense secretary try to micro-manage the military services by focusing on one little program after another rather than focusing on how to build a coordinated defense force. The result, Nunn said, was evident in Grenada where the Army and Navy brought different types of radios, preventing soldiers and sailors from talking to each other during the battle.