IN THE WAKE of the Marines' casualties in Beirut, President Reagan is being asked to submit the question of their tenure to Congress under the terms of the War Powers Act. He is resisting. Yesterday Secretary of State Shultz said that although the 1,200- man force ashore had come under fire and had returned fire, it was not engaged in actual combat: hence the president is not required to ask Congress to review its continuing presence. This is a pretty legalistic answer, one unlikely to calm congressional and public anxieties about a violent situation on full view on the nation's television screens.

The fact is that the premises on which the Marines were sent to Beirut last year are in sad shape. The official view then was that from the wreckage left by the Israeli invasion, at least one good result--a free, united and unoccupied Lebanon--could be extricated. It was precisely the expectation that the Marines would not be drawn into battle, but would merely stand behind the growing authority of the Lebanese army as the Lebanese government extended its sway through the national territory, that won the Pentagon's reluctant assent. The president's own optimism shone through his reference to "the settlement in Lebanon"--actually, just the evacuation of the PLO --in his speech of Sept. 1, 1982, announcing that he was moving on to address a broader Mideast peace.

A year later, Syria's troops remain in Lebanon, defying American diplomacy. Israel is turning its back on the massive contribution its invasion made to Lebanon's political maladies. In the latest battles around Beirut, Druze and Shiite Moslem militias are exploiting the Gemayel government's predicament to press their claims for revision of Lebanon's basic political structure--the very claims that underlay the civil war that began in earnest in 1975 and never really stopped. The expectation that the Marines' stay would be short-lived and safe is no more.

It would be shocking if the United States were to turn tail because two Marines were killed. But President Reagan cannot possibly want to have the force's presence in Beirut become a contentious issue, which it surely will if he does not explain in specific and convincing terms just what diplomatic strategy its presence is serving and what steps are being taken to minimize risks to its members. What would, and should, most disturb Congress is that a president has at least appeared to put American servicemen into a dangerous place without having a clear idea of what they are doing, and of what he is doing.