There is a considerable community of people who believe that this is a moment of crazy and accidental, but great and unprecedented, promise for an Arab-Israeli settlement and that President Reagan's peace proposals, besides being fair and balanced, supply the long-missing piece of the puzzle: American commitment. Yet among serious and experienced observers who feel that way, a strain of pessimism lurks not far beneath the hope.

It is not simply that, as everyone knows, the situation is murderously difficult, local political options are circumscribed, distrust is rife, the basic condition of flux gives extraordinary play to chance, and so on. All that is a given.

No, the problems are two: the first arises from the definition of the Mideast stalemate commonly accepted in the Arab world and progressively more accepted here. It is said to be a matter of the United States' laying down the law to Israel, not permitting it to get away with its intransigence anymore.

But let us suppose -- and many people do -- that Reagan's chosen method of infinite solicitude for Israeli security fails to produce the desired results and finally he is driven to put on the squeeze: by opening up a line to the PLO, suspending aid and the rest of it. In certain circumstances, a troubled but resolute Reagan may well be carried to such a pass by the thrust of his own commitment.

But what would be the consequences of such a showdown? At best, uncertain. At worst, humiliating verging on disastrous. Reagan or a successor could yet reach the end of the road, having given it his all, and look up five days or five years later to find that there had been another war, or something, and the Israelis were still sitting on the Palestinians in the West Bank. Not even full American abandonment of Israel -- something I cannot imagine, by the way -- could ensure the success of the Reagan plan. The question would then be whether the costs justified the effort.

The second difficulty lies in the nature of Arab politics, or Arab society. In a sense, the Palestinians' leadership, meaning the PLO, may be the most legitimate, the most reflective of the community it presumes to represent, of any Arab political entity. But collectively all the Arab leaderships remain narrowly based, inauthentic to a degree, lacking not only popular mandates but institutional bulwarks, overwhelmed by the multiple weights and confusions of modernity, scared to death of internal disruptions, hardly fit vehicles for the rude political choices required on the Arab side to resolve the Arab-Israeli dispute.

In brief, the Israelis have it together and the Arabs do not. The Israelis are organized, and they are organized around a concept of security with potentially deadly implications for compromise with their neighbors, and the Arabs are disorganized, fumbling still for a concept, a manner of modern adaptation, that will let them cope with the frightening rush of change.

There is a school of thought in Israel that knows well the sources of Arab inadequacy and is prepared to capitalize on them. The strategy dictated by this cold awareness is to rely on Israel's discipline, technology and American connection and to use the Arabs' own vulnerabilities against them.

It is why a leading scholar of Arab politics, Georgetown's Michael Hudson, can ask, voicing a suspicion widely if quietly held in the Arabist community, whether the Palestinians may not already have lost their struggle for a West Bank homeland: the land is being taken, the Palestinian dispersion continues apace; it is all over except for the tears and the bombs.

It is difficult for people who want peace to believe this but they -- we -- dare not overlook the considerations that may make it so.

It makes one wonder whether our soft continental experience has adequately prepared Americans for the global tasks we continue to be ready to assume. Other nations, from older, wearier corners of the world, wonder whether our innocence is disqualifying, or merely burdensome. But they know, too, that circumstances and their own frailties make them dependent on our choices.

We take a great responsibility upon ourselves. We are the wiser for seeing the pitfalls, but we cannot afford to be paralyzed by them.