The War Powers Resolution -- enacted into law over a presidential veto nearly nine years ago -- has never been more relevant than it is now with U.S. Marines in Beirut for the second time in slightly more than one month. Relevance, however, is but a law's whitened sepulcher if it is not being complied with fully. No wonder that questions persist and doubts arise about whether or not War Powers is being adhered to properly.

As with most laws woven with the fragile thread of words, there can be intense but honest differences about definition and meaning. Inevitably, there will be even greater disagreement over the precise nature of the circumstances in which the law is to be applied, especially in a situation as fluid and volatile as Beirut. Finally, one must balance that often delicate nuance of the "spirit" of the law against its rigid "letter."

My own assessment of the situation in Beirut based on information from official U.S. government sources suggests that the Marines, who went into the Lebanese capital on Wednesday, were entering an atmosphere where "imminent involvement in hostilities is clearly indicated by the circumstances." These words directly out of Section 4 (a) (1) of the act are crucial because it is only a 4 (a) (1) report that triggers the balance of the act's congressional action provisions.

Rather than such a 4 (a) (1) report, however, President Reagan's report Wednesday was only "consistent with" the War Powers Resolution. In so doing, he avoided starting the 60-day clock at the end of which the troops would have to removed. More important, however, he also precluded Congress from providing a specific statutory authorization or otherwise acting on the issue within the formal terms of the law.

In the process of thereby avoiding full and proper compliance, President Reagan--like presidents Ford and Carter before him -- is eroding the integrity of the law. By succumbing to the advice of the slick State Department lawyers, Mr. Reagan is weaving amid the shadows of the law and avoiding its full spirit of intent. As a result, we may soon be faced with a constitutional crisis.

Although the death of one Marine and injury to three others less than 24 hours after entering Beirut was not, strictly speaking, a result of hostilities, the incident did point up the pervasive dangers present in the city. The persistent volatility, uncertainty and instability within the Lebanese capital was more dramatically and tragically demonstrated by the massacre of hundreds of innocent civilians in the West Beirut refugee camps. Also increasing the prospects of imminent danger is the fact that the Marines are stationed within the city and are, therefore, more publicly visible and susceptible than they were in August when limited to the port area. Finally, the indefinite and explicit vagueness over the length of the mission underscored in my mind the need for careful monitoring and control. When taken into full account, all of these facts clearly demand the need for congressional concurrence.

What was lost by not seeking congressional involvement in this decision?

Obviously ignored was the very purpose of the War Powers Resolution, which is stated in Section 2: ". . . to fulfill the intent of the framers of the Constitution . . . and insure that the collective judgment of both the Congress and the president will apply to the introduction of the U.S. Armed Forces. . . (emphasis added).

I believe that by cutting Congress out of that decision, Reagan also undid his own best interests. In his singular action, the president runs the hazard of unilaterally stumbling into a possibly intractable U.S. military involvement. That potential could well prove to be tragic beyond even the pain of American casualties by leading to a rapid and serious erosion of American popular support for his recent and promising Middle East peace initiative and of Israel itself.

Finally, by precluding the kind of informed and open congressional debate on the possibility of a statutory authorization for the troops going into Beirut, Reagan deprived the nation of the opportunity to better understand--and ultimately more strongly support--the decision.

What we expect of the president, however, the Congress must impose upon itself as well. The effectiveness of the War Powers Resolution or any other law depends on full executive branch compliance as well as on the determined and vigilant oversight of the Congress.

A Congress that is truly responsible where War Powers is concerned cannot cry wolf every time an incident involving U.S. Armed Forces occurs. We must also not see "hostilities" around every darkened corner. We must be fair, prudent and circumspect in our review of situations like Beirut. Not to exert such balanced restraint can ultimately undermine the integrity of the act as quickly and surely as does limp compliance by the executive.

The worst possible condition would be a War Powers Resolution made meaningless by either congressional or executive branch abuse. A War Powers law made sterile through over-expectation or eva sive and casual compliance may not be effective when we need it most.

Today, as U.S. Marines attempt to bring peace and avoid further war in Beirut, is the time for the president to recognize that the War Powers Resolution is the law of the land.