The physical and social devastation of Lebanon after Israel's mauling of the Syrians and Palestinians (to say nothing of what the Lebanese had earlier done to one another, or the PLO and Syrians to them and to each other) is great.

At a guess, it is not less extensive than what the Union armies did to the South in 1864-65, or the Allies to Germany 80 years later. And we know how long repairs took.

Yet, we are told, from the ashes of Lebanon there is to spring miraculously a stable and coherent phoenix-state in which, as The New Republic puts it, "Lebanon can now be Lebanon."

Israel is pushing this happy vision, not without some calculated flattery to U.S. statecraft.

We are asked not just to help pick up the pieces but to play at the great game of state- making that began, in its modern phase, with Sykes and Picot. They were the diplomats who secretly divided French from British interests in the Mideast during World War I, and the process has continued with various commissions, panels, treaties, inquiries and committees ever since.

An American administration with no clear policy on the Middle East may find the task formidable. But the Reagan administration is at least unencumbered by certain sterile legalisms that inhibited its predecessors. Sometimes it is better not to know just what you think about complex matters than to think lucidly but erroneously--as the Carter administration did in 1978 when it hotly condemned Israel's earlier incursion into south Lebanon. It thus indirectly helped establish a sanctuary for a PLO military buildup that made Israel's second, more crushing, invasion a matter of time.

But assuming that the Israeli invasion is as successful as it looks, what might it mean to say that "Lebanon can now be Lebanon"? Israel wants the Syrians out and the PLO irregulars disbanded: desirable, but politically speaking only a beginning.

Lebanon, from the post-World War II Anglo- French withdrawal to its collapse in civil war eight years ago, was always more a precarious arrangement than a nation.

It was governed much as New York City was half a century ago. Its fragile political coherence was the product of a nicely calibrated distribution of political and military privileges among incompatible factions.

Maronite Christians, who did not get on well with Orthodox Christians, shared power and place with Moslem factions united only by fear of Christian dominance. It now seems Israel's hope that Lebanese Moslems will accept a restoration of Christian paramountcy unseen in Lebanon since the days of the French mission civilisatrice. It is unlikely.

A second hope springing from the ruins and ashes is that Israel's mauling of the PLO will, or could, eliminate it as a factor in Mideast politics--a result devoutly to be wished.

Yet the painful truth is that the PLO is an obnoxious expression of a substantial political reality: the quest for ultimate justice to the displaced Palestinian Arabs. That reality shows no sign of being vulnerable to guns, rockets and tanks.

All of which is another way of saying that whether Lebanon can be Lebanon depends on two things: what Israel decides to be, and what it will permit the Palestinian national idea to become.

The limits of Israel's national aspirations were once fairly clear. But clarity faded with the coming of Menachem Begin and the Likud. Begin, with his Old Testament mysticism about Greater Israel and a growing Sephardic constituency with hard-line views about Arabs, obscured the old limits.

So long as Palestinian nationalism finds expression in violence, and in the declared aim of extinguishing the Jewish state, Israel need not confront the ultimate issues of its nature, size and destiny: whether Israel is to be a realization within defensible and peaceful borders of the vision of Herzl, Weizmann and Ben-Gurion, or a miniature Middle Eastern Prussia, ruling subject Arab populations by blood and iron in the name of an ever more elusive security.

PLO obduracy and hostility have made possible an Israeli policy in which the windfalls of war harden into de facto gains, and de facto gains (outside the pre-1967 borders) harden into juridical claims, such as the recent "annexation" of the Golan Heights.

Lebanon's chaos is in large measure a reflection of Israel's own crisis of identity. If the first can be fixed without a clarification of the second, it will be the miracle of the century.