The compromise reached between Congress and the White House over development of President Reagan's cherished ''Star Wars'' program was a modest victory for Sen. Sam Nunn.

The administration agreed not to take any action during the next fiscal year on the Strategic Defense Initiative that would do violence to a traditional interpretation -- Nunn's view -- of the 1972 U.S.-Soviet Antiballistic Missile Treaty. The White House claims that a broader interpretation of the ABM Treaty would allow the United States to develop ABM weapons and test them in space, contrary to the traditional -- more restrictive -- view.

Actually, the state of SDI research and development is such that no tests could possibly be conducted before the end of Reagan's term. So the White House wasn't giving up very much in its compromise agreement.

In the long run, though, the compromise may prove to have given decisive support to the narrow interpretation of the ABM Treaty. According to this view, espoused by Nunn, who is chairman of the Armed Services Committee, testing of new weapons in space could not be carried out by either side without abrogating the pact.

Nunn's ''strict construction'' of the ABM Treaty was reinforced, according to our sources, by columns we wrote on Reagan's secret decision to go ahead with development -- and testing -- of a chemical laser weapon. The hush-hush project was given the code name Zenith Star.

As we reported, Reagan himself initially accepted the traditional interpretation of the 1972 agreement: that developing such a weapon, and particularly testing it in space, would constitute a violation. But in an Oval Office meeting last Dec. 17 with Defense Secretary Caspar Weinberger and Lt. Gen. James Abrahamson, head of the SDI program, Reagan ordered ''full speed ahead'' on the chemical laser weapon, which would be able to shoot down incoming Soviet missiles before they reached American air space.

According to reliable accounts, Weinberger told the president that the ''Alpha'' hydrogen-fluoride laser under development by TRW since 1980 was the best bet for an early testing of an exotic Strategic Defense Initiative weapon.

Reagan noted that the Zenith Star program would eventually violate the ABM Treaty as he understood it. The key section of the treaty is Article V, which states that neither signatory will ''develop, test or deploy ABM systems or components that are sea-based, air-based, space-based or mobile land-based.''

Despite this seemingly unambiguous language, the president ordered Abrahamson to proceed anyway.

Our sources say Abrahamson recommended that Zenith Star ''be done openly'' and that in effect the Soviets should be given the six months' notice required by the treaty if either side intends to abrogate it.

Reagan rejected the general's suggestion that Zenith Star be made public. He explained that Congress was already at his throat over the Iran/contra arms scandal, and the political price he would pay for endangering the ABM Treaty ''would be too high.'' As for notifying Moscow, Reagan tabled the recommendation for later consideration.

Meanwhile, he ordered that Zenith Star be a ''black'' or super-secret project, which meant Congress would not be kept informed of either its goal or its progress.

Abrahamson did as he was told, and the first phase of Zenith Star, a five-month study by Lockheed, Martin Marietta and Rockwell, was begun last January. The current, second phase is a three-month, $10.8 million effort to design a fully integrated space-based laser weapon. The contract announcements carefully disguised the true nature of the Zenith Star component programs.

Reagan's secret decision reflected his intense desire to have at least a prototype space weapon that could be tested before he leaves office -- thus demonstrating to scoffers that his SDI program is not just a pipe dream. But he has reluctantly accepted the reality that the very earliest Zenith Star could possibly be ready to test is late 1990 -- and then only if there are no delays due to technical problems.

The president was bitterly disappointed. As one aide told us, Reagan ''would like to demonstrate one of these exotic weapons tomorrow if he could.'' Instead, he has settled for an expedited Zenith Star project, evidently in the belief that the more progress made by Jan. 20, 1989, the harder it will be for his successor to scuttle the program.

No matter how close to completion the laser ''death ray'' is by the end of Reagan's tenure, he will be leaving the next president a dead-certain donnybrook with Congress. Whichever way the new president decides to go -- scrapping either Zenith Star or the ABM Treaty -- he will arouse the wrath of some powerful senators.