Washington — As US marines prepare to enter Beirut, US officials and Middle East experts are striving to look beyond the tangles of Lebanese politics to the resolution of the crucial Palestinian problem.
''If we wait until all the problems in Lebanon have been presumably solved, we'll wait forever,'' says Joseph Sisco, former assistant secretary of state for the Middle East.
''The time has come for us to take the initiative,'' Mr. Sisco told the Monitor. ''We have to begin to explore and put forward concrete, substantive ideas designed to break the impasse rather than continue the essentially passive postman role that has been played over the past year and a half.''
In recent days, President Reagan, Secretary of State George P. Shultz, and Defense Secretary Caspar W. Weinberger all have acknowledged as much. US officials are working on what Mr. Weinberger calls an ''American plan'' for solving the Palestinian problem.
Middle East expert William Quandt of the Brookings Institution notices a marked difference in US signals on the region.
''I think it's pretty clear that there is a different attitude from the one that existed say three or four months ago,'' he says. ''I think there is more of a determination to put the United States into a position of some leadership on these negotiations. Quite frankly, since about the middle of 1979, we've taken a relatively back-seat role.''
At his first press conference, Secretary Shultz said, ''The language of Camp David, as I read it, has lots of room for ideas as to how the situation might be arranged.''
It remains to be seen if this will lead to a significantly different position on such difficult issues as eventual Palestinian self-determination, which the secretary of state concedes has become ''a term of art.''
''He feels that out of the ashes of the dispute, we should move with dispatch ,'' State Department spokesman John Hughes said Monday. ''But I can't say he has a scenario.''
Secretary Shultz has been seeking the counsel of members of Congress and such well-known former government experts as Henry Kissinger and Sol Linowitz.
But if the US is seeking wider and more imaginative solutions within the Camp David accord signed four years ago, it will find this difficult, given the present attitudes of Egypt and Israel.
In a communique issued over the weekend, Israeli Prime Minister Menachem Begin said Israel ''will initiate action'' for a comprehensive peace, but stressed that ''there will be no negotiations on any proposals whatsoever which deviate from the framework for peace established at Camp David.''
In a Washington Post column, however, Egyptian President Hosni Mubarak insisted that ''the United States must recognize the right of the Palestinian people to self-determination.'' This goes beyond the specifics of the Camp David accord. Egypt says it won't take up the broader questions until Israel leaves Lebanon, and Israel says it won't leave until Syrian troops pull out.
Thus, just as the United States is about to begin a more active diplomatic role in the Middle East, the other two participants at Camp David apparently are hardening their positions and demanding clear disengagment among the forces in Lebanon.
Mr. Quandt of the Brookings Institution feels the US will initially direct whatever new action it takes at the Arab side of the Middle East equation.
''We're not going to start pounding the Israelis immediately until we have a pretty good assurance that there is an Arab party to the negotiation,'' he says. ''I think they'll spend some time seeing what can be done to convince the Arabs that we're serious and that they should get serious.''
Meanwhile, however, experts like Quandt and those within the US government acknowledge that ''there's plenty to be done just in Lebanon, and whether we like it or not, we're right in the middle of it.''
Not far off the coast of Lebanon, 800 US marines are cleaning their M-16s and checking their ammunition supplies for what American officials hopefully insist will be a noncombatant role in Beirut this week. In addition to their personal rifles and pistols, the marines will be equipped with machine guns, mortars, and antitank weapons, but no tanks or artillery. Not far away, however, will be the aircraft carrier USS Independence, which is heavily armed with attack and fighter aircraft able to reach Beirut in minutes.
Pentagon officials say that the marines' role is ''pacific,'' that they expect ''no interaction'' between US and Palestinian forces. Individual marines will have the right to decide for themselves, however, when and if to return fire if they come under attack - even if this should come from Israeli soldiers.