SOMETHING HAS BEEN happening in the

political arena in Lebanon that brings into focus exactly what is wrong about an Israeli operation that has gone as far as it has in good part simply because people have been unable to keep up with it.

Keep in mind that the prime rationale for the invasion was to substitute a viable authority--at first, Israel's--for the long-missing or at least weak central Lebanese authority, so that Lebanon would no longer remain a base of terrorism. On this basis, many Americans initially tended to accept the Israeli invasion, notwithstanding their dismay over its heavy civilian toll.

In Lebanon this week, in a true political miracle, a central Lebanese authority was assembled. Leaders of the principal Lebanese communities came together, in a "National Salvation Council," for the first time since civil war shattered the government in Beirut in the mid-1970s. They adopted a plan meant to spare Beirut the catastrophe of a final Israeli assault. It called for withdrawal of Israeli forces to six miles outside the capital, the return of PLO soldiers to refugee camps and the stockpiling of their arms, and the insertion of the Lebanese army as a buffer.

In other words, the Lebanese did exactly what Israel had demanded: under incredibly adverse circumstances, they came together and drafted a plan formalizing at once the sovereignty of Lebanon and the reduction and mortification of the PLO.

The Israelis then spat on them, rejecting the plan and insisting further that Yasser Arafat and the PLO "core" leadership be expelled to a country not bordering on Israel.

We are forced to several conclusions, of which the first is that the Israelis have misled the world about the purposes of their invasion. Dealing with terror seems now to have been the tritest part of it, although this is the platform from which the Israelis trolled for sympathy or at least forbearance. Nor was Israel concerned simply with humiliating Syria or with lording it over Lebanon. Its deeper now-revealed intent is to take advantage of a favorable regional opening--and of Ronald Reagan's inattention or quiet consent, whichever it is--to impose a Begin-type Palestinian solution. That means destroying the Palestinians as a movement and dispersing them as individuals.

A second conclusion is that the Israelis have been unforgettably bloody. There is no conceivable goal that would justify the civilian casualties that a further, full-fledged assault on the PLO and Syrians in Beirut would surely bring. That would be the ultimate barbarism in a campaign in which the widespread slaughter of civilians has undermined Israel's claim to moral superiority, once its richest political asset in the West. Again, the only thing possibly more astonishing than the ruin Israel has wrought is President Reagan's silence in the face of it.