No one wants to embrace Chicken Little as a role model, but isn't it at least possible that the latest noise in Lebanon was the sound of the sky falling?

Maybe the White House and the State Department can convince us that Monday's naval bombardment--not to protect our Marines but to provide direct military support for the embattled Lebanese Army--was only a barrage of acorns. Maybe they will be able to make the case that the escalation of our military involvement in order to prevent the collapse of the Gemayel government makes sense.

But the fear here is that from the moment our warships, the John Rodgers and the Virginia, opened fire on the Syrian-backed Druze and their Palestinian allies, it was too late for us to do anything sensible in Lebanon--maybe even too late to get out without serious, perhaps disastrous, consequences.

Maybe it's been too late for a year. The first Reagan dispatch of 800 Marines to Lebanon, in August 1982 made sense; the mission was clear and limited: to maintain order during the withdrawal of Syrian and Palestinian forces from Beirut after the Israeli siege of that city. The Marines, as part of a three-nation force with the French and the Italians, accomplished their mission and came home.

The mission was substantially less limited or clear when, a bit more than two weeks later, the multinational force, including our Marines, was back in Lebanon--this time at the invitation of the Lebanese government in the wake of the massacres in the Palestinian refugee camps of Sabra and Shatilla. It sounded clear enough. The stated mission was to "restore Lebanese government authority over the Beirut area." But given the notorious weakness of both the Lebanese government and its army and the multiplicity of forces contending for power there, it might appropriately have been dubbed "Mission Impossible."

Nor is that only the 20/20 vision of hindsight. The obvious question occurred to a lot of us at the time: what happens when they start shooting at our guys?

President Reagan answered that question last month when our Marines did in fact come under fire. The answer: shoot back.

At that point, even some who had accepted the logic of the original peace- keeping purpose started to glimpse the nature of the problem. Shooting back when shot at makes sense, but sitting there waiting to be shot at doesn't. If you think you know who is shooting at you, or is planning to, doesn't it make sense to initiate preventive strikes? And if that happens, haven't your forces been transformed from peace- keepers to combatants? The seeming inevitability of being drawn far deeper into the actual conflict than our national interests justified prompted a lot of Americans to demand that our troops be brought home. And quickly. For if we waited until they were heavily involved in the fighting, taking casualties and allowing themselves to be cast as alien heavies, extrication would take on the appearance of panicky flight.

As dismaying as that scenario was, Monday's shellings made it worse. For the first time, our troops were not firing back in self-protection but launching strikes in behalf of one of the combatants in a civil war. The official rationale is that we had to do it in order to ensure the safety of our troops and diplomats and to avoid the catastrophe that would ensue if the Gemayel government should fall. In other words, the only way we can keep the peace is to win (or help to win) the war.

That's not what we bargained for. The immediate dilemma is clear: just as it becomes obvious that we need to get out of Lebanon, this week's escalation makes our quick withdrawal diplomatically, militarily and perhaps even morally impossible.

The sky has fallen, and nothing makes sense.