Assuming as well as assigning responsibility for our acts must characterize a mature society. It is in that light rather than in mere faultfinding or scapegoating that the congressional inquiry into the Oct. 23 Beirut truck-bomb tragedy, in which 241 US servicemen were killed, should be viewed.

In its early findings, the House Armed Services Committee investigations panel has confirmed what seemed obvious: that ''very serious errors in judgment'' were made by officers on the scene and up the chain of command. Intelligence warnings of possible terrorist attack, specifically by explosives-laden trucks, went without response. Personnel trained to monitor such intelligence were not provided. Security protection was lax.

More broadly, apart from a lack of vigilance on the site and of oversight by commanders in the region, the Washington decision to position the Marines at the airport - ultimately a Pentagon and White House decision - put the troops in unusual jeopardy. Warning of further casualties ahead, the panel urged the Reagan administration ''in the strongest terms'' to consider whether further Marine deployment is necessary to its Lebanon policy.

Responsibility for the lives of servicemen, so many of them still in their teens, is a serious matter. A public identifies with its servicemen as parents and friends, not just as abstract symbols of national defense and pride. It is not a military charge alone the public entrusts to its government when sons and fathers leave home.

Many analysts had warned the administration that the Marines were not the contingent best suited to a ''peacekeeping'' role in the first place. Marines are combat troops. The kinds of personnel better suited to a situation where military intelligence and political considerations are paramount, it was countered, might have given the impression the US was settling in for a longer haul. As it is, the local commanders are regularly recycled, giving little continuity to the leadership.

Lack of preparedness for terrorism has dogged the United States in the Middle East for some time, from the US Embassy seizure in Tehran in 1979 to the recent truck-bomb attack on its embassy in Kuwait. Fault doesn't lie in the Pentagon alone. The State Department's months-long delay in defending its Kuwait mission, after intelligence warnings, shows that bureaucratic chain-of-command processes routinely can fail to respond to an alert. The US seems determined to see irrational terrorism and local political forces largely in conventional terms, to its repeated cost.

Ironically, the Beirut tragedy review comes at a time when the administration and Congress are considering focusing more direct responsibility for military operations in the military command itself. Repeatedly we hear Washington defer to local commanders' assessments of what is needed to defend positions or carry out missions. The use of US air strikes in Lebanon and decisions to fire battleship batteries are publicly cast as appropriate local military responses.

It may be convenient for those charged with civilian oversight of the military - which is the way the US Constitution begins the chain of command - to ascribe responsibility down the line and out of Washington. However, the US position in Lebanon, from the diplomatic scurrying down to the troops bunkered in Beirut, reflects nothing but administration policy, which Congress with full deliberation has endorsed. Hence the anguish on the House panel among Republicans and Democrats alike over the vulnerabilities inherent in US policy in Lebanon.

The terrorists who attacked the marines are the primary culprits. And local commanders' errors can frustrate or ruin an otherwise sound strategy.

But within the US the perception grows that ''Marine diplomacy'' bears an inherent contradiction: American peacekeeping neutrality was sacrificed when US militia were seen as enforcers of President Gemayel's position; the length of the Marines' stay no longer is material to that perception. No assigning of blame down the line to local commanders can relieve elected officials and the public that elected them of the sense of responsibility for US policy.

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