On Friday, Sept. 17, Israeli Ambassador Moshe Arens and Prime Minister Menachem Begin skipped their regular midnight call. It was Rosh Hashanah, the Jewish new year, and the beginning of the High Holy Days.

So it was that the first Arens knew of the Beirut massacre was from the newspapers he read before he left for the synagogue on Saturday.

"At that point," Arens said later, "there really was not much more than an inkling. It didn't sound good . . ."

After the five-hour worship service at Adas Israel Synagogue in Northwest Washington, Arens and his wife, Muriel, joined Rabbi Stanley Rabinowitz for a small, private luncheon at his home.

"At the luncheon table we talked about the massacre at length," recalled Rabinowitz.

"The ambassador was obviously distressed and we didn't press him. The facts were not in."

During lunch, Arens was summoned to the telephone and told that Secretary of State George P. Shultz wanted to see him at the State Department. When Arens left, he went directly to the embassy and called Defense Minister Ariel Sharon.

That was when he "got the first information about the dimensions of the tragedy and the circumstances of it," he said.

The meeting with Shultz lasted nearly 30 minutes, and afterwards, Arens drove home, where he again telephoned Israel. This time he spoke with Foreign Minister Yitzhak Shamir, passing along Shultz's demand that Israel withdraw from Beirut immediately. Throughout the evening he worked with his staff on the official statement to the press, which began, "Israel expresses its dismay and shock at the killing in Beirut . . . ," and he concurred with a decision to turn down a television appearance the next morning on ABC's "This Week With David Brinkley."

"We figured it was better to wait for more information," said press counselor Nachman Shai.

For the second night in a row, despite the rumblings of a growing storm in Israeli-American relations, the Israeli ambassador went to bed without speaking to his prime minister. It was the second night of Rosh Hashanah.

In his seven months here as Menachem Begin's ambassador, Moshe Arens has weathered some stormy moments in Israel's traumatic history: the return of the Sinai, the invasion of Lebanon and now the Beirut massacre. Today, Yom Kippur, the day of atonement for Jews the world over, Israel finds itself in a crisis of confidence, with deep unrest at home and its "policies, image, character, values and aspirations," as former foreign minister Abba Eban has put it, "less understood and admired . . . than in any other period of her history."

Arens, who is only the seventh ambassador from Israel to the United States, its most steadfast ally, talked about the massacre in an interview last week.

"We have before us an event of unparalleled tragedy," he said, sitting in his second-floor office at the Israeli Embassy. "Since the creation of Israel in 1948, I don't remember one that was itself so terribly painful and the cause for such horror."

As Israel's man on the spot charged with explaining the actions and policies of the increasingly controversial Begin government, the 56-year-old Arens has earned a reputation for coolheaded credibility. He is a political hawk who eschews the emotional for the more clinical approach in interpreting Israeli events and policies. Yet that day, as accusations against the Begin government were mounting, even he seemed somewhat shaken.

"We've never had that kind of tragic event, with the ramifications and the things that were said between the United States and Israel, between Americans and Israelis," he said. "In that sense it's a singular event in relationships."

Asked whether he thought Begin would resign if it turned out that a member of his cabinet was implicated, Arens replied, "I doubt it." But, he added, "If in parliament it became clear to a majority of members . . . that the government had not acted effectively, that the prime minister had not, it would bring the government down."

Unhesitating in his American English, perfected as an adolescent growing up in New York City's Washington Heights, he rushed on.

"We don't have all the details . . . I think there is every reason to believe it was stopped as early as it possibly could have been stopped . . . I don't know if it will become apparent that some sergeant knew about it maybe six hours before the Israeli army went in to stop it. But it's not a sergeant who can stop this sort of thing . . . It's almost extraneous to say that nobody else did anything . . . the United States Marines had left, the French Foreign Legion had left. Had it not been for the action of the Israeli army, I guess it would have been far more horrible, if we can imagine such a thing."

Whatever the circumstances, Arens continued, he was certain that in Israel all the facts would come out.

"It's a very free and open society," he said, perhaps an understatement in light of the massive demonstrations that soon followed in Israel. Expressions of outrage and demands for an independent investigation have shaken the country and culminated in the largest protest rally in Israel's history.

Arens' performance since his arrival in Washington has earned him admiration. Despite his businesslike approach -- some have called him humorless -- he is regarded as an ideologue with a streak of independence and a firm grasp of Begin's policies.

"He's under enormous pressure and in great demand, yet there is an ease about him, a neatness in how he handles things. I've seen him field a lot of questions, yet people don't feel threatened," said Warren Eisenberg, a director of the B'nai B'rith International Council, one of a number of American Jewish groups in recent days seeking clarification of Begin's policies.

"He is very straightforward, very tough and never beats around the bush," said ABC-TV's "Nightline" anchorman Ted Koppel. "He is also there to protect Israel."

Arens was a guest on Koppel's show the night Lebanese President-elect Bashir Gamayel was assassinated.

Koppel: "It's been suggested that this creates something of a vacuum in Beirut and Israel may now feel the need to move in and fill that vacuum."

Arens: "It isn't Israel's job to move in and fill that vacuum. It is Israel's objective as it is the American objective, as I understand it, that an independent, sovereign Lebanon be reconstituted . . ."

Koppel said later that either Arens did not know the Israelis were at that moment moving in on West Beirut or he was not going to say. "But I don't think any ambassador ever comes on 'Nightline' unless he perceives it in his country's best interest."

The Israeli Embassy called it "a misunderstanding" and then began to limit Arens' television and radio appearances on grounds that it was "premature" for him to give interviews. Between Sept. 9 and Sept. 24 he had made nine appearances on television and two on radio.

Arens plays down American misunderstandings about Israeli actions and policies, but accepts some blame for those that have occurred.

"If there's a misunderstanding, then certainly the people who are responsible for transmitting messages in both directions carry a good share of the responsibility. That's myself, the United States ambassador in Israel and some of the other actors in the drama, though I don't mean to lay any blame here on Sam Lewis U.S. ambassador to Israel , who is a good friend of mine," said Arens.

He cited two recent examples:

Israel never intended to say that its military operation in Lebanon would be limited to an imaginary line drawn 40 kilometers north of its borders.

Neither did Israel intend to make an absolute commitment that its army would never enter West Beirut.

He said that in negotiating the evacuation of Palestine Liberation Organization guerrillas from West Beirut, nobody foresaw the assassination of the Lebanese president-elect, or raised the question of whether under such circumstances the Israeli army would move in.

"So you can begin to make some excuses for the people involved," he said. "A lot of misunderstandings have to do with second-guessing future circumstances that really are not fully envisaged."

Arens, who describes himself as "a dilettante talking about a profession he's brand-new at," nevertheless has formed some strong opinions about that profession -- diplomacy.

"From what I've seen so far of diplomacy, I don't think it's much of a profession in the sense that aeronautical engineering is a profession. I wouldn't let any guy get near designing an airplane who hadn't studied aeronautical engineering and had reasonably good experience in designing pieces and parts of airplanes. I don't get the feeling that it's the same thing in diplomacy."

Designing airplanes, in fact, was how Arens spent most of his adult life. Visitors to his embassy office are reminded of that by three models of Israeli planes whose development he supervised between 1962 and 1971, when he was vice president and chief engineer of Israel Aircraft Industry.

He was born in Kaunas, Lithuania, in 1925 and lived in Riga, Latvia, until he and his family fled to the United States in 1939. Though his parents were "not particularly" Zionist, he said he decided as a child that he would go to Israel after World War II.

"Riga probably had the highest percentage of Jews exterminated of any community in Europe, more than 95 percent. So you can imagine that that made some impression on me," he said.

One of about 200 in the local Betar Zionist youth movement, Arens entered the Massachusetts Institute of Technology in 1943, went into the U.S. Army in 1944 and was getting ready to go into combat when the Americans dropped the bomb on Hiroshima. Reentering MIT after the war, he finished in 1947 with a degree in mechanical engineering, then against his family's wishes went off to join the fight for an independent Israeli state.

"They said there was no better place in the world than the United States," he said. "But your views and ambitions don't always agree with your family, and my views and ambitions are pretty continuous with my background and impressions."

In Palestine, he joined the Irgun. He also met Menachem Begin.

"I was in some awe of him as commander of the underground," said Arens.

By 1951, after working on a border kibbutz, he had returned to the United States to study aeronautical engineering. With him was his American-born wife, the former Muriel Eisenberg, whom he had known in New York City.

"She wasn't there by accident. She went over to join the Israeli Army," he said. They have four grown children, two studying in the United States and two in Israel.

At the California Institute of Technology he earned his degree in aeronautical engineering, then went to work in the aircraft industry in New Jersey. By 1957, he was back in Israel, where he taught at the Technion in Haifa and later joined Israel Aircraft Industry, in charge of developing systems that included the Kfir and Lavie fighters.

"I could go back tomorrow," he said, and indeed probably could, since he assiduously keeps up with the industry through its literature and periodicals. He is, he says, always making suggestions about airplanes to the powers that be in Israel.

He views diplomacy, as he views aeronautics, with the systematic mind of an engineer who seeks answers not just to present-day causes and effects, but to future ones as well. "Engineers," he noted, smiling, "have not been known for making great successes on political scenes -- Herbert Hoover, would you consider him one of your best presidents?"

Jimmy Carter, he said, once sat in on a meeting of the Knesset Defense and Foreign Affairs Committee, of which Arens was chairman.

"I could see he was an engineer like me. He had a yellow pad and as people were speaking he was making his notes in very clear handwriting with typical shorthand notations that engineers use," Arens remembered. "But then some people tell you that when you're that systematic about things you lose the overall picturee, you don't get the full perspective."

In fact, it was the Carter-initiated Camp David agreement spelling out peace terms between Egypt and Israel that kept Arens from accepting Begin's first job offer, that of Israeli defense minister in 1980. Opposed to some of its concessions, though not to peace itself, Arens voted against the treaty in the Knesset, where he had served since 1974.

As an outspoken and hard-line member of Begin's own Herut party, Arens' visibility had been growing. In July 1978 he pushed for "all necessary action" by Israel to prevent a takeover of Lebanon by Syria, warning that if Christian forces were destroyed, a new frontier would be created on Israel's border. In August 1978, Arens charged the Carter administration with providing "at least tacit support" to the Syrian presence in Lebanon.

In June 1981, after the Israeli Air Force knocked out a 70-megawatt nuclear reactor near Baghdad, Arens called the operation "justified" and the American reaction to it "unfortunate".

In December 1981, as ambassador-designate, Arens raised a few eyebrows when he said the United States had adopted Saudi Arabian views toward the Middle East crisis and that this had caused a deterioration in U.S.-Israeli relations.

When word reached Washington that Arens would be the next ambassador, there was some apprehension about his tough-talk stance. In Israel, some people were saying the only reason he got the job was because he spoke English.

"Offhand," Arens noted drily, "I wouldn't consider myself particularly qualified to deal with the media because I'm an engineer. An engineer is supposed to be kind of screwy."

As much as anyone, Menachem Begin helped put Moshe Arens where he is today, and despite differences at times, theirs is a close relationship.

"It's easier for him to communicate with me because we know each other very well," said Arens. "That's one of my advantages, and it stems from the fact that I am not a professional diplomat. I come out of the body politic in Israel, out of Begin's party. I guess he feels communication with me is easy because he understands what I have to say and he can trust me with what he has to say."

"I never expected to be a diplomat," he said. "This was thrust upon me. I've been an engineer all my life."

Is he glad he entered the political arena?

"Tell you in a few years," he said.