In an extraordinary secret order, President Reagan declared that if Congress failed to provide satisfactory funding and support for his Strategic Defense Initiative, he would abandon the traditional interpretation of the U.S.-Soviet Antiballistic Missile Treaty, which has been accepted by every president since the treaty was signed in 1972.

The secret document, National Security Decision Directive 192, which Reagan signed in August 1985, is crucial to an understanding of the showdown that has been building up since last spring between the White House and Sen. Sam Nunn, chairman of the Armed Services Committee. Suspecting that Reagan was planning to abandon the ABM Treaty through an exercise in semantics, Nunn and Sen. Carl Levin sponsored a bill that would prevent the president from implementing any reinterpretation of the treaty without congressional approval.

The secret 1985 directive -- which members of Congress were never meant to see -- makes clear that the senators' suspicions were right on the mark.

''Recently, we reexamined the ABM Treaty and the associated negotiating record in great detail,'' Reagan's directive stated. ''In the process, we have gained new insights into how this treaty can objectively be interpreted.''

As we reported recently, Reagan acknowledged in a secret Oval Office meeting last December that a chemical laser development program, code-named Zenith Star, would probably violate the ABM Treaty eventually. But he ordered the program to be expedited anyway.

Instead of risking a fight over ''reinterpretation'' with Congress, which was in an uproar over the Iran-contra scandal, Reagan ordered Zenith Star to proceed in absolute secrecy. Congress was to be kept in the dark.

The potential for disagreement over interpretation was fairly obvious. The treaty's Article V states: ''Each party undertakes not to develop, test or deploy ABM systems or components which are sea-based, air-based, space-based or mobile land-based.''

Recognizing the possibility of futuristic weapons such as lasers, the treaty added Agreed Statement D, which pledged both sides to have further discussion if research proceeded on ''ABM systems based on other physical principles'' than those enumerated in the treaty. In short, space-based testing of laser weapons was implicitly banned.

This traditional, ''restricted'' interpretation of the treaty was followed by Presidents Nixon, Ford and Carter -- and was accepted by President Reagan as well until his secret 1985 directive.

The first hint that Reagan had begun to rethink his position came with testimony before the House Armed Services Committee on Feb. 27, 1985, by Lt. Gen. James Abrahamson, the SDI chief. While assuring the members that the SDI program was then in compliance with the ABM Treaty, he added: ''At some point, however, we would have to depart from the treaty.'' He foresaw that occurring ''about the turn of the next decade.''

Over the next few months a secret review of the treaty and the negotiations that led up to it was conducted with an eye toward reinterpreting it more broadly to permit tests, including laser tests. This resulted in the ''new insights'' mentioned in the secret directive of August 1985.

One insight was that ''our initial and unilateral assertions about what the ABM Treaty did restrict concerning advanced defensive technologies is not clearly demonstrable in the terms of the treaty as written, nor in the associated negotiating record.''

While piously proclaiming that SDI research had conformed to the narrower, traditional interpretation of the treaty, Reagan's directive closed with this:

''I can envision that in the future the day will come when our research will have answered the questions necessary to permit us to consider going beyond the restrictions that we have and will continue to observe under the current research program. At that time . . . the United States will have the opportunity to reassess the guidance that I have set forth in this document.''

The secret directive laid the theoretical groundwork for reinterpreting the ABM Treaty. From there, it was but a step to Reagan's order last December to proceed apace with the Zenith Star laser program.