Once upon a time, Ronald Reagan and his friends from California enjoyed a happy thought. They would come to Washington and apply to the tangled problems of the Middle East the idea of a ''strategic consensus.''
The ''strategic consensus'' would bring together in happy concert the Israelis, the Arabs, and the Americans. Together they would keep the Soviets out of the Middle East and away from the pipelines that carry the oil of Arabia to the homes and factories of the industrial democracies.
This week, a year and a month after the people from California reached Washington, that once happy concept is only a memory. It has been dissolved in the intractability of the Arab-Israeli conflict.
That conflict is once more near a flash point. Israeli armed forces are mobilized for a possible incursion into Lebanon. They are held back largely by pressure from Washington that at any moment could prove insufficient.
The result is a new level of tension between Washington and Tel Aviv. And this has been heightened by the recent trip to the Middle East by US Secretary of Defense Caspar Weinberger. During that trip a person on his plane (officially unnamed but possibly Mr. Weinberger himself) was quoted as saying that the United States is trying to ''redirect'' the emphasis in its Middle East policy away from Israel toward the Arab states.
This expression was underlined by reports from the Weinberger trip that he was offering modern US weapons to the Arab states he visited. Also, Mr. Weinberger visited Saudi Arabia, Oman, and Jordan on his trip - but omitted Israel.
The idea that Washington was preventing Israel from taking ''preventive'' action against the forces of the Palestine Liberation Organization (PLO) in southern Lebanon, and at the same time apparently offering modern F-16 fighters plus mobile surface-to-air Hawk missiles to Jordan, set off a tempest of anxiety in Israel.
Prime Minister Menachem Begin made an impassioned speech in the Knesset and obtained 88 votes in favor of a resolution expressing ''profound concern'' over the proposed sale to Jordan. It called on the United States to refrain from ''thus gravely imperiling Israel's security.'' It gathered an almost unanimous vote of the Knesset.
It also prompted a letter from President Reagan to Mr. Begin that left matters in as ambiguous a condition as they had been before the letter was penned.
The letter repeated the familiar Reagan promise to sustain Israel's ''qualitative and quantitative'' advantage over its Arab neighbors. But it also asserted the desirability ''for both our countries'' for the US ''to enhance its influence with other states in the region.''
How does Washington enhance its influence with the other states in the region , all of which are Arab, without persuading them to buy US weapons rather than Soviet weapons? And, if the Arab neighbors of Israel continue to build up their strength, will the time not come when Israel will no longer possess both ''qualitative and quantitative'' superiority over its combined Arab neighbors?
Jordan has recently announced an intention to buy some weapons from the Soviets. If it is unable to obtain the weapons it wants from its accustomed sources in the West, it will probably turn to Moscow as the best alternative source of supply. And that is precisely what Washington wants least of all to happen.
In this particular case, the Jordanians are interested in mobile Hawk missiles. They already have fixed Hawks. But those do not greatly worry the Israelis. Their jets can take out the fixed Hawks.
But enough mobile missiles could evade their detection and some day deprive Israel of that air supremacy in its neighborhood that has been a main ingredient of its military superiority. The Israelis were promised during the Ford administration that the US would sell only the fixed, not the mobile, Hawk missiles to the Jordanians.
Promises made to Israel by the Reagan people during the 1980 presidential campaign and during the early months in the White House led Mr. Begin to dream a dream of his own: that the US would base its military strategy in the Middle East on Israel and on bases in Israel; that, in effect, Israel would become America's only active ally in the Middle Easd.
That Begin dream has also collapsed, along with the original Reagan dream of the ''strategic consensus.''
Mr. Reagan continues to assure Israel of Washington's devotion to Israel's security and survival. But he has repeatedly criticized Israel for doing things Israel says it had to do for its security. The exchanges have included censure for the bombing raid on Iraq's French-built nuclear reactor and the bombing raid on the high-rise apartment houses in Beirut that allegedly housed some PLO offices.
And when Israel announced what amounted to annexation of the Golan Heights, Mr. Reagan suspended implementation of the military security agreement between the US and Israel that had just been negotiated.
The officially unidentified spokesman aboard the Weinberger plane said that the US could not any longer be a ''hostage to Israel.'' The words have been officially brushed aside. But it is perfectly clear from the events of the past week that there is a growing gap between actual US operating policy toward the Middle East and what Mr. Begin thought he had been promised by Mr. Reagan.
Mr. Reagan continues to assert total US devotion to the safety and security of Israel. But his administration is not allowing Israel itself and alone to define what is necessary to guarantee that security. For instance, every possible US effort is being made right now to dissuade the Israelis from the invasion of southern Lebanon that Israel's military high command argues is necessary to prevent the buildup of PLO forces in Lebanon to the point where Israel could no longer take them out at will.
A diplomat attempting to identify US policy toward the Middle East at this stage of the Reagan administration would be justified in putting it in the following terms:
Reagan declaratory policy favors Israel and identifies Israel as an ''ally'' of the US. But US operating policy is veering around toward that posture of ''evenhandedness'' between Israel and the Arabs which was US policy during the Eisenhower era and was preached if not always practiced from then until Mr. Reagan took over the White House in 1981.