The attack on United States marines in Lebanon raises anew serious questions about American forces around the world. Are they ''spread too thin,'' as Sen. Sam Nunn (D) of Georgia and other experts worry? What are the practical limits to military power when used as a tool of diplomacy in complex situations like the Middle East or Central America? Is there a basic and perhaps dangerous mismatch between US national security goals and the military resources available to meet them, as some senior military officers are privately warning?
As rescuers sift through the rubble at the Beirut airport looking for survivors and US officials search for ways to prevent another suicide attack, such broader issues are being discussed here. They are at the heart of the debate over the Pentagon budget. And they point up the seeming paradox of senior military officers urging caution in the use of US forces abroad.
''I think there are just fundamental limitations to military power,'' said Col. Harry G. Summers Jr., who teaches military history at the Army War College.
''We're seeing military power in Lebanon as an extension of our diplomatic efforts, and of course that's a legitimate use of military power,'' says Colonel Summers, a decorated veteran of two wars and author of an acclaimed book on Vietnam. ''But in this situation the power potential or force potential is just not too relevant.''
''It's almost a no-win situation,'' he continues. ''If the President keeps them (the marines) there, they're going to continue to be a target for this sort of thing. But if he withdraws them, then he destabilizes the very thing he set out to do.''
Other analysts are concerned that US forces are currently unable to fulfill every task assigned them. In recent weeks, American forces have increased their alert status in Korea, headed for Grenada, exercised in Honduras and Europe, trained government troops in El Salvador, been aboard warships cruising off the Central American and Lebanese coasts, shown the flag in the Persian Gulf before steaming toward the Philippines, and traded shots with snipers in Beirut.
The Congressional Budget Office reported earlier this year that the Rapid Deployment Force - established in 1979 after the Soviet Union invaded Afghanistan and now called the US Central Command - could in effect weaken NATO by drawing away forces intended for Europe. Critics, including presidential candidate John Glenn (D) of Ohio, say the new organization is ''neither rapid, deployable, nor a force.''
''One indication of how thin we're spread is when you look at what was immediately available for reinforcements,'' said one congressional expert. ''One reason they couldn't keep combat marines off the coast of Lebanon was because of the shortage of amphibious shipping that was needed elsewhere.''
''You can't be in Grenada and you can't be sailing to the Philippines and you can't be off the coast of Lebanon all at the same time with the same ships,'' this source says.
In essence, this echoes earlier warnings from the Joint Chiefs of Staff that even the Reagan administration's largest-ever peacetime military budget would not cover everything the armed services were being asked to do. And it follows Defense Secretary Caspar W. Weinberger's persistent question to Pentagon budget critics: What diplomatic commitments and strategic interests - if any - should the US renounce?
Meanwhile, the more immediate inquiry focused on the future role of the marines in Lebanon, whether their rules of engagement should change, and how to protect them against possible future attacks.
There is a reluctant realization here among most observers and participants that the marines will have to remain in a visible role in Lebanon, although some in clerical and other support positions may be shifted to US Navy ships offshore.
The marines' rules of engagement allow them to call in naval gunfire and attack aircraft to defend their positions. Naval shelling already has been used, but there is great hesitancy to call on ship-based aircraft for two reasons: the intermingling of Lebanese civilians with the forces attacking the marines, and the close proximity of Soviet-made Syrian antiaircraft batteries that could draw the two superpowers closer to armed conflict.
Speaking to broadcasters and editors at the White House Monday, President Reagan strongly emphasized that he is not considering withdrawing the marines from Lebanon. He also might increase the number of troops there, he said, if the Marine Corp commandant, Gen. Paul X. Kelly, (now in Beirut) determines that this is necessary to carry out the marines' peacekeeping mission.
But the President stressed that the marines should not be actively engaged in combat in Lebanon.