With Israel poised to attack West Beirut, the United States seems close to achieving some major Middle East policy goals but is on the verge of compromising its long-term national interests and those of its moderate Arab friends.

This seeming paradox stems from an open American unwillingness to rein in the Israelis in time to prevent their invasion from dissipating their clear successes and sowing the seeds of future regional turmoil.

So far the Israelis have served American interests by humiliating the Soviets, who not only provided the Syrians with inferior weaponry but also failed to play a superpower role in the defense of either their Syrian or Palestinian friends.

Equally, the invasion has cut down to size what recently seemed to be the triumphant march of militant Islam symbolized by the battlefield victories of Iran against Iraq.

Some enthusiasts even argue that an overall Middle East peace process could move ahead following events of the past several weeks and that Lebanon could be put back together. That reportedly was the message given by President Reagan and Secretary of State Alexander M. Haig Jr. to Israeli Prime Minister Menachem Begin in Washington this week, when they apparently said that Washington had no disagreement with what Israel was doing in Lebanon but wanted some movement now in "the peace process."

But Western diplomats and Arab analysts suggest that unless a way out is found for Yasser Arafat and his Palestine Liberation Organization--which seems increasingly unlikely--and unless a way is found to save West Beirut from Israeli ground attack, then the United States and its moderate allies will end up paying the heaviest part of the bill in the aftermath.

If Arafat feels obliged to die fighting to keep the Palestinian cause alive for the future, the Arab world seems sure to seize upon his martyrdom as a perfidy specifically committed by the Americans in arming and encouraging the Israelis. Arafat himself already has expressed this idea.

Palestinian terrorism, whose destruction the Israelis have invoked as a goal to justify the invasion, is then likely to blossom as never before, uncontrolled and uncontrollable against American as well as Israeli targets. Palestinian moderates say an indiscriminate assault on West Beirut would result in everybody but the terrorists being killed.

And if the Israelis destroy a good part of West Beirut in their assault--a plausible assumption in light of the massive firepower they employed against Tyre, Sidon and Damour--the number of civilian casualties, already very high, could easily double.

Those who suggest that the Israelis would be satisfied with capturing the Palestinian camps--especially Sabra, which is the nearest thing that the Palestinians have to a capital--assume that the Palestinian guerrillas and their Lebanese allies inside the city proper would surrender. The probability, however, is that the guerrillas would keep on shooting, and the Israelis would use planes, artillery and tanks to dislodge them block by block, street by street.

Responsible Lebanese politicians believe that reconstructing this tormented country, a major aim proclaimed by Israel and the United States in this war, would be impossible for years to come in the wake of bitterness left by more Moslem dead and the Christian Phalangist militia's de facto alliance with Israel. And the United States would be tagged with responsibility for that aspect of the invasion.

Embarked on a show of strength toward its Arab neighbors, Israel seems to be according little priority to such considerations. American officials in the region and in Washington have at times suggested that, on the contrary, Israel has systematically sought to frustrate U.S. relationships with moderate Arab regimes such as Jordan and Saudi Arabia and prefers to be confronted in the region by more radical regimes that cannot look to Washington for any support.

Thus, a possible radicalization of the region does not appear at this stage to be a major deterrent for Israel. But potentially high casualties inherent in street fighting have served as a brake in delaying the ground assault against West Beirut.

Israeli statements have made it clear that Jerusalem would prefer to use its overwhelming military presence to force the Palestinians and their Lebanese allies into unconditional surrender without having to resort to such a potentially costly attack.

But the complexities of Lebanese politics appear so far to have been more intractable than military problems for the Israelis.

It is now 10 days since Israeli Defense Minister Ariel Sharon led his troops into Baabda, the seat of the last remnant of Lebanese sovereignty. The Israelis have sought to incorporate their Maronite Christian militia ally, the Phalangists, into the battle, and prod the Christians into finishing off the Palestinians for them. The Phalangists have resisted that temptation, however, wary of compromising their position with other Lebanese factions by associating too closely with the invaders.

Paradoxically, many Lebanese recognize that without the Israeli military pressure, there is little chance of forcing change in the Lebanese political scene, with its hatreds and pettiness encrusted after more than seven years of violence.

But they add that time is desperately needed to bring the Lebanese Moslems around to abandoning the Palestinian cause, which they feel honor-bound to defend regardless of their inner hatred for the often overbearing and violent guerrillas.

"The Israelis will end up attacking West Beirut, because no quick political solution is possible," he sighed. "This is the first time that the Israelis have had to deal with the Palestinian problem in Lebanon, which has baffled Lebanese presidents for the past 15 years."

These Lebanese keep hoping that some face-saving formula can be found, but they are convinced that the Israelis and Americans are not interested at all.

In theory, they argue, the Palestinians could--if given time to explain things to their troops--give up their heavy weapons. Israel already seems assured at the very least of a 25-mile-deep demilitarized zone in southern Lebanon, which would put Israel proper beyond range of any guerrilla guns.

But the Israelis have Arafat in such a corner that going down fighting--and adding even more martyrdom to the Palestinian cause--now seems the only way to respond to demands for unconditional surrender without outside guarantees of protection for him and his followers if they disarm.

So 500,000 West Beirutis sweat it out, hostages of the Palestinians, who in turn are hostages of the Israelis. And many Lebanese are concluding that history will record that the United States was also a hostage of the Israelis in this affair.