U.S. officials regard the expected choice of Moshe Arens, Israel's ambassador here, to replace Ariel Sharon as defense minister as an essentially positive sign that the impasse over withdrawing Israeli troops from Lebanon can be resolved without a major U.S.-Israeli confrontation.

That assessment seems outwardly at odds with Arens' image as a hard-line Israeli nationalist in the mold of Prime Minister Menachem Begin. Arens publicly has opposed U.S. ideas about the nature of a Lebanon solution and President Reagan's broader peace initiative for resolving the Arab-Israeli conflict.

However, in the U.S. view, Arens also says he believes strongly that continued close ties with the United States are a bedrock necessity of Israel's foreign policy and that, when points of friction arise as in Lebanon, it is imperative to reach an accommodation with Washington.

That is regarded here as the crucial difference between Arens and Sharon. Within the administration, Sharon was seen as intent on using bullying tactics to force the United States into a position of continuing to supply Israel with arms while giving the Jewish state free rein to determine what actions best serve western interests in the Middle East.

For that reason, even before Tuesday's release of the Israeli report on the massacre of Palestinian civilians in Beirut, U.S. officials had carefully weighed the possible effects on American policy goals and had concluded tentatively that the best result would be one that toppled Sharon, while enabling Begin to retain power.

Now, at a time when events appear to be moving toward Sharon's ouster or shift to a lesser Cabinet post, the expectation that Arens will emerge with the defense portfolio--a position that would make him the front-runner eventually to succeed Begin--fits this best-case scenario of U.S. policy makers.

No one in the administration is under any illusions that the change will lead to a quick breakthrough on a Lebanon agreement. In fact, the expectation is that the current turmoil unleashed within Israel by the report will set matters back perhaps by several weeks.

In an interview with The Washington Post last week, Arens, while stressing that the United States and Israel have "a common objective in Lebanon," warned that his government intends to keep pressing its demands in the Lebanon negotiations and stressed that reaching an accord "is going to take some time."

Still, the U.S. hope is that the removal of Sharon's dominant presence from the Begin Cabinet will produce a new power center there focused on Arens and foreign minister Yitzhak Shamir, whose views closely parallel those of the ambassador, and that they gradually will nudge Begin toward coming to terms over Lebanon.

If that proves to be the case, some U.S. officials noted yesterday, it will be the most important result of the year that Arens has spent at the Israeli Embassy.

By that, they mean that the experience gave him a keener, firsthand understanding of the fact that U.S.-Israeli relations have been deteriorating seriously and that, even within the American Jewish community, there are growing pockets of concern that Begin's aggressive policies could be leading Israel into collision with U.S. interests.

That has not always been apparent in Arens' public stance, even though he grew up in the United States and has spent much of his life here. He is an aeronautical engineer turned politician who became a power in Begin's rightist Likud bloc and who, when he served as chairman of the defense and foreign policy committee of the Israeli Knesset, opposed the Camp David agreements.

In fact, when Arens became ambassador a year ago, the Israeli government made known that he was coming as a signal that it intended to be much more aggressive in reinforcing Begin's determination to pursue his view of Israeli interests and to resist U.S. pressures.

In the administration and in diplomatic circles, the reviews on his performance have not been particularly favorable. His most effective moments have come during periods of acute tension when his articulateness has made him an impressive spokesman on television and in public appearances for the Israeli position.

But, while Arens has said many times that he wanted to improve "communication and understanding" between Washington and Jerusalem, most observers feel that he usually has had the opposite effect.

In particular, he is regarded to have put a belligerent, highly politicized stamp on the embassy to the point where it currently is not regarded as a major channel in the dialogue between the two governments or an especially reliable interpreter of American attitudes to Jerusalem.

In the current issue of Foreign Policy magazine, an article by Zeev Schiff, military editor of the Israeli newspaper Haaretz, contends that Israel's invasion of Lebanon last summer was given an implicit green light by the United States. That argument has been disputed strongly by U.S. officials, and the Schiff article is based primarily on deduction rather than any hard evidence.

However, the article does make the point, well known in diplomatic circles, that there was considerable ambivalence in the positions conveyed to the Israelis before the invasion by then-Secretary of State Alexander M. Haig Jr., and that the Israeli embassy here was bypassed frequently in communications between the two capitals, a situation that allowed Sharon and his supporters to argue their version of Washington's views within the Israeli Cabinet.

Now Arens, who was frequently described here as "a square peg in a round hole," appears headed for a position where U.S. officials hope he will be working with a far better understanding of American attitudes and thus will be able to halt the growing rift in the longstanding U.S.-Israeli relationship.