Bracing itself against an expected tide of international condemnation for annexing the Golan Heights, the Israeli government today appeared to have willingly approached -- if it has not already passed -- a watershed in its relations with the United States.

Beneath the private assurances by Israeli officials that the furor over the annexation will soon pass are signs of awareness that Israel's relations with the United States, which has strongly protested the action as a violation of the Camp David accords, may be more stretched on this issue than ever before in the four-year tenure of Prime Minister Menachem Begin.

In response, as always when confronted by criticism for a stand it perceives as fundamental to the existence of the Jewish state, the Begin government has decided to do what it appears to think works best: employ public relations.

In Hebrew, it is called hasbara when the purpose is to reshape public opinion abroad. At its best, hasbara was set in motion by a bipartisan parliamentary delegation earlier this year in an effort to turn U.S. opinion against the eight-point peace plan advanced by Saudi Arabia; in a less successful form, it was mobilized to try to convince Congress not to sell airborne surveillance and enhanced long-range strike equipment to the Saudis.

Today, the Israeli Foreign Ministry announced that it would launch an intensive information campaign abroad to try to explain the government's decision to annex from Syria the 16-by-42-mile plateau of the Golan Heights that commands an imposing and strategic position above the fertile Galilee Valley.

Deputy Foreign Minister Yehuda Ben-Meir, in disclosing the public relations initiative, which will include personal contacts with a number of foreign ministers by Israeli Foreign Minister Yitzhak Shamir, said that Israel is not underestimating the enormity of its task.

"We knew we were facing a political struggle. We knew we were facing an information struggle. We are gearing to meet this struggle. Certainly, we will put up a good case and a good fight," Ben-Meir said.

In the corridors of the Knesset, the parliament, there has been worried talk about the indignation of the U.S. ambassador to Israel, Samuel Lewis, who normally prides himself on reading Israel's intentions accurately and keeping Washington relatively well insulated from surprises.

Lewis is said to have been as surprised as everyone else about Begin's decision to push the Golan Heights bill through the parliament on the day of his discharge from the hospital, where he underwent an operation for a broken hip joint. The U.S. envoy is also said to have been personally hurt that Begin left him in the dark, as he did his entire Cabinet until just hours before the annexation vote.

But Begin, in a summation speech to the Knesset last night, underscored a principle to which he has adhered unwaveringly in his 33-year career in Israeli politics: on paramount issues of security, Israel should be willing to risk even the good will of its staunchest ally.

That principle was reiterated tonight by Shamir, who, in an interview on Radio Israel, said he was sorry if the United States government felt hurt by the surprise annexation decision by the Cabinet.

"But, as much as we want to coordinate our activities with the United States, the interests are not identical. We have to, from time to time, worry about our own interests."

Then, in a comment that appeared to reflect the Begin government's confidence that the furor will subside, Shamir added, "In the past, there have been misunderstandings between Israel and the United States, and after a period of time, things have returned to normal." He said he hoped this would happen in the "near future."

While Israeli officials continued to couch the annexation law in terms of "applying Israeli law" to the Golan, they acknowledged that Begin's declaration that the heights had become part of the "land of Israel" implied the outright assertion of Israeli sovereignty over the Syrian territory.

Legal observers noted that the language of the Golan Heights bill is identical to that used by the Knesset in June 1967 when it annexed East Jerusalem and part of the West Bank, and that Israeli courts had interpreted the application of Israeli law to an occupied territory as giving Israel sovereignty over it.

Taking into account that world opinion would recognize the language of the Golan Heights bill as a euphemism for annexation, Israeli government sources cited what they said they regard as compelling reasons for going ahead with the unpopular move. They are:

* Rejection by the recent Arab League summit in Fez, Morocco, of a Saudi Arabian peace plan that only obliquely suggested the right of Israel to exist, and Syrian President Hafez Assad's reported assertions that even if the Palestine Liberation Organization recognizes Israel's right to exist, Syria will not.

This argument, while appearing to overlook Israel's outright rejection of the Saudi peace plan when it was first announced and Assad's longstanding refusal to recognize Israel, is the centerpiece of the Begin government's declared justification for annexing the Golan Heights.

* The internal pressure on Begin's Likud coalition to fulfill promises made in the campaign platform and in coalition agreements. Officials also noted that there is an overwhelming consensus in Israel for annexation of the Golan.

* An opportunity to capitalize on U.S.-Libyan friction and blunt the Reagan administration's reaction to annexation, because of reported moves by Libya to send "hit squads" to assassinate U.S. officials.

* An opportunity to capitalize on the diversion of international attention toward the crisis in Poland, and the realization that the Soviet Union might be too preoccupied by events in Poland to rush to the aid of its client state, Syria.

Some Labor Party critics of the timing of the annexation law suggested today that the Soviet Union might seek to divert world attention from Poland by coming to the assistance of Syria and creating a Middle East crisis.

The opportunity to test Egypt's intentions to continue with the autonomy negotiations and normalization of relations. Also, there is a feeling among some observers that Israel would have faced a stronger reaction by Egypt if it had awaited to annex the Golan after the April 25 Sinai withdrawal.

The Labor Party opponents of the annexation move -- or, more properly, of the decision to make the move at this time -- said that possible negative results included further Arab pressure on European nations not to join the U.S.-sponsored Sinai peace-keeping force; the possible involvement of the Soviet Union in the dispute; jeopardizing the autonomy talks and further normalization with Egypt and the solidification of the fractious Arab world into a more united front against Israel.

Asked tonight what Israel would do if Syria expressed interest in negotiating a return of the Golan, Shamir said, "Yes, Israel has said it is willing to negotiate everything. We do not set preconditions."

But he emphasized he did not foresee the return of the Golan.