IS A GRAND irony now taking place in the Middle East? Israel invaded Lebanon not simply to crush the PLO as a military force but to put it out of business as a political force--the better for Israel then to impose its own cramped brand of autonomy in the West Bank and Gaza. But unexpected turns in the war may be setting the stage for a resurgence of the PLO as a political force in ways confounding Israeli war plans and offering vistas no one else had imagined.

The most obvious evidence of the PLO's new lease on political life is its participation in the Beirut talks with the governments of Lebanon, the United States and Israel. These talks concern not just lifting the city's siege but determining--that is, ensuring--the PLO's organizational continuity. It was not possible for the Americans and Israelis to have a voice in working out the first task without lending legitimacy to the second. No less than the Lebanese, however, Americans and Israelis have accepted the PLO as a working partner in their mind-boggling common enterprise. Who would be surprised to learn down the road that American, and perhaps even Israeli, negotiators saw their Palestinian counterparts coming or going and perhaps had words?

In Washington, the war has freshened the sense that the Palestinian question is central in the Middle East and that the PLO unquestionably represents the Palestinian people. Already the question has arisen of whether the United States is slipping off its longstanding pledge not to "recognize or negotiate with" the PLO unless it accepts Israel first. The policy does not appear to have changed, though certainly it should if PLO evolution makes it possible. In any event, the policy has been shown not to stand in the way of exchanges of the sort now being conducted in Beirut.

The connection, though indirect, is open, and through it the United States is taking a leading role in ensuring that if all goes reasonably well the PLO will emerge unbowed though bloodied from the war and that its political identity will be preserved. In public words, furthermore, President Reagan, while disdaining the "armed PLO," is plainly trying to draw the political side of the PLO into concessions that would permit both direct American dealing and PLO participation in the Camp David procedures.

In this regard, the PLO's reaction to the prospect that American forces might be sent briefly to Beirut to help cover its departure is interesting. The symbolism of rescue by a force identified in PLO propaganda as Israel's patron is offensive to Yasser Arafat, but he understands well the practical value of rescue and he has shown himself alert to the possibilities of using the operation as a key to broader, more overtly political dealings with the United States.

Many people are still saying--and therefore in a sense anticipating and condoning--that the PLO's only post-Lebanon option is further terror. But that outcome is not fated. The blessings of the peacemaker will fall on politicians who hold open another choice.