The US marines who returned to Beirut this week are a larger, more heavily armed force than the one that left just 19 days ago. In addition, they are assuming a mission that is more difficult and of less certain duration than the previous one.
Shortly after the first contingent of marines arrived in Beirut Wednesday, a Pentagon spokesman said their purpose would be ''to enable the Lebanese, in a serene atmosphere, to be able to perform the normal police functions necessary for the successful operation of the city.''
On Tuesday, President Reagan said the marines will remain until foreign forces leave Lebanon. He pronounced himself ''reasonably optimistic'' that both Israel and Syria would do so quickly. (There are approximately 30,000 Syrian and 50,000 Israeli troops still in Lebanon.)
This leaves less certain the question of the thousands of Palestinian fighters still in Lebanon. ''There's no question that they are included within the scope of what we're looking for in terms of the withdrawal of foreign forces ,'' said a State Department spokesman.
How willing will Israel be to leave Lebanon knowing that PLO forces remain? Such questions are making many in the Pentagon nervous.
''It's more open-ended than we anticipated it was going to be,'' confided one militry officer. ''We assumed it was going to be based purely on events centered in Beirut. Now, when the President threw the entire country into it, including the Israelis, including the Syrians and the PLO up in the Bekaa Valley, that was very surprising here. So it is extremely open-ended.''
Under the US War Powers Act, Congress must approve any deployment of American forces for more than 60 days. This leaves the President some latitude in determining how long US forces should remain if there are no hostilities or hostilities are not imminent.
In any case, plans now are being made to relieve that part of the Marine Amphibious Unit now in Lebanon, which is due to return to the United States early in November.
Some 800 marines went ashore in Beirut Wednesday and anothwr 400 are expected to arrive today.
During their last tour there, the marines were confined to the port area. This time, they will be based at the airport on the southern edge of the city and patrol north to the presidential palace, securing and occupying positions at intersections.
Last time they carried only light arms. Now they are equipped with several tanks, armored personnel carriers, mortars, and antitank weapons. Their general orders are the same as earlier - to defend themselves if fired upon. But there is no talk in Washington now of quickly withdrawing the marines if any more than sporadic combat incidents occur.
While there may be some contradictions in Washington's official pronouncements about how long the marines may remain in Lebanon, there is no doubt that they are prepared to remain for considerably longer than their first tour. Also, they are being backed up with US Sixth Fleet forces, including the aircraft carrier USS Independence. The carrier's air wing includes fighter and attack aircraft loaded with weapons and standing by not far from the Lebanese coast.