What Israel has long feared has come to pass: The United States, its strongest ally, and the Arab states, its sworn enemies, have come forward with peace proposals, leaving Israel on the sidelines shouting no to any negotiations except on its own terms.

Within the past two weeks, the government of Prime Minister Menachem Begin has been cast in the role of the lonely naysayer, rejecting not only the substance of the U.S. and Arab positions but even the possibility of using them as a starting point for negotiations.

Today, the Israeli Cabinet unanimously described the proposals adopted at the Arab summit conference in Fez, Morocco, last week as "hostile" and designed to bring about "the destruction of Israel."

The swift and total rejection was predictable. The key Arab positions -- Israeli withdrawal from all the territory it captured in the 1967 war and the creation of an independent Palestinian state with East Jerusalem as its capital -- are completely unacceptable not only to the hard-liners in the Begin government but also to the vast majority of Israelis and to the Reagan administration.

But the mere fact that the Arab states for the first time agreed on a common plan was a setback for Israel, which long has benefited from disarray and discord within the Arab world.

Less than a year ago, alarm spread through the Begin government when President Reagan murmured some cautiously approving comments about the eight-point peace plan advanced by then-crown prince Fahd, now the king of Saudi Arabia. The Israelis feared a further tilt by the United States toward Saudi Arabia and a gradual isolation of Israel from its traditional friend and ally.

"The floating of the Fahd plan," the Jerusalem Post observed at the time, "has been viewed as Saudi Arabia's own opening shot in a 'peace offensive' apparently designed to isolate Israel internationally by putting forward calculatedly unacceptable proposals and forcing Israel into a rejectionist pose."

But then the Arab states, as they had before, came unwittingly to Israel's rescue. An effort to achieve general Arab support for the Fahd plan at the 1981 summit conference in Fez collapsed hours after it began.

The failure of that summit, Israeli officials promptly announced, was a vindication of their argument that Saudi Arabia is not a major political factor in the Middle East and that the Arab world is incapable of collectively coming to terms with the existence of Israel.

The positions adopted by the Arab states last week in Fez, according to Israeli officials, are "even worse" than the Fahd plan because a section implying willingness to recognize Israel has been weakened. But the combination of a first unified Arab negotiating position -- coming on the heels of the Reagan initiative, which in turn followed the summer-long war in Lebanon and the dramatic siege of Beirut -- has put Israel in the awkward position of appearing to be a country that has waged war with great success but is reluctant to discuss peace.

There appears to be a general recognition of this within the Israeli government. Briefing reporters today, an Israeli expert on Arab affairs called the Fez declaration a "tactic" designed to "drive a wedge" between the United States and Israel.

He said the Reagan initiative had encouraged conservative Arab nations such as Saudi Arabia to accept the positions and language of the radical states for the sake of presenting a united Arab front.

Even so, according to this official's assessment, Israel should have nothing to worry about in the long run. "Sooner or later President Reagan must read the text of the proposals," he said, and note all the "negative elements" in them.

Nevertheless, in the rapidly changing atmosphere since the fighting in Lebanon, there are developments of concern to the Israeli government.

If Begin was alarmed by Reagan's comments last year, he could not have been pleased by Secretary of State George P. Shultz's initial assessment that the plan adopted at Fez could be a "genuine breakthrough" if it implies willingness to recognize Israel.

Nor could Begin be happy about the signs of erosion in support for his policies in the American Jewish community, such as a statement by Thomas Dine, executive director of the American-Israel Public Affairs Committee, that the Reagan proposals have "a lot of value."

These developments appear to have helped provoke Begin's outburst last week in which he accused the American media and unnamed administration officials of plotting to use Reagan's initiative as a lever to bring down Begin's government and replace it with a presumably more accommodating leadership from the opposition Labor Alignment.

Begin's taunting threat to call early elections next spring over the future of the occupied territories began to evaporate today. The Labor Alignment's Central Committee decided not to take up the challenge, saying that if the prime minister really wants elections he can arrange it on his own by having his government resign.

Meanwhile, Begin promised the other parties in his government coalition -- at least two of which oppose elections next spring or earlier -- that they will have an effective veto over any early elections decision.