Until last week, Ronald Reagan seemed to stand aloof from the political mantraps of Middle East diplomacy. But modesty was in order. During the 1980 campaign, his advisers discovered that Reagan's grasp of the issues at play there scarcely went beyond visceral and sentimental support for Israel.

He was, for instance, unfamiliar with the critical "Black September" of 1970, when King Hussein's Arab Legion (with Israeli tanks poised to keep the Syrians out of the struggle) crushed the Palestinian refugee militias. These events led directly to the repudiation of Hussein as spokesman for the Palestinians and, eventually, to the war of expulsion in Lebanon.

But the Israeli operation in Lebanon once again dramatized, for all to see, that the United States is hostage to Middle East fortunes.

Reagan, after some hesitation, has seized the 1977 Camp David agreement as the only promising avenue to peacemaking. No other seems to have a prayer, and even in Camp David's case it is only a prayer.

Several European initiatives since Camp David have been contemptuously rebuffed by Menachem Begin. Having surrendered oil wells, with other strategic assets, for peace with Egypt, he can hardly be expected to sympathize with those who think oil when they say "Palestinian rights."

Unfortunately, one attraction of the Camp David "framework" is that, in making it, the late Anwar Sadat and Begin agreed to set aside the really nettlesome issues.

Thus, nothing was written into the agreement about (a) the steadily expanding Israeli settlements in the West Bank, or (b) the annexationist impulse to which they (and recent pronouncements by Defense Minister Ariel Sharon) unmistakably point, or (c) the future status of the city of Jerusalem. These issues were, at Camp David, relegated to the never- never land of "negotiation among the parties."

Begin and Carter, it soon developed, had in mind very different futures for the West Bank. And PLO terror discouraged any Arab resident disposed to cooperate in the creation of the councils. So five more years have slipped by. And today Israel is even more resolutely entrenched in the West Bank and Jerusalem than at the time of Camp David.

The novelty of the Reagan proposals lies not in the proposals themselves (a settlement with Jordan has been talked about for years), but in the president's willingness to outline, even sketchily, a blueprint for peace.

Under the Reagan plan, the West Bank would be neither Israeli, nor Palestinian (in the sense of a sovereign state), but a buffer zone under "self-government by the Palestinians . . . in association with Jordan."

The exact meaning and prospects of this formula are as uncertain as ever. Israel's and Jordan's maps for the future of the region match only in the paradoxical sense that each regards the other as "Palestinian" in having been carved from the original mandate.

The critical question, probably, is whether Hussein, the last Hashemite king, is any more enthusiastic than Menachem Begin about the return of his sometimes armed foes to an "entity" to be situated on the West Bank.

Now that the PLO guerrillas are dispersed, disarmed and presumably demoralized, Israel is the unquestioned master of the situation -- for a time at least. No other party has the will or means to break the impasse. If, as every recent sign indicates, Israel under Begin plans to pursue its vision of security at the expense of large Arab populations and at the risk of the world's displeasure, continuing deadlock seems in prospect.

If Israel and Jordan do agree to meet in Reagan's sensible halfway house, it will be a miracle. But miracles do happen, even (or is it especially?) in the Middle East.