President Reagan and his Cabinet were lining up on the White House South Lawn shortly before 10 a.m. Monday to greet Mexican President Jose Lopez Portillo when deputy press secretary Larry Speakes came out and quietly motioned to Secretary of State Alexander M. Haig Jr.

The first bulletins were coming across the wires informing the world of what senior U.S. oficials had known secretly for the preceding 20 hours: Israel had used American-supplied jet fighter-bombers to destroy the Iraqi nuclear reactor installation under construction outside Baghdad.

However, before Haig could do anything, a musical announced Lopez Portillo's arrival, and Haig found himself a prisoner to the ceremonial necessity of standing in the sun while the two presidents exchanged speeches about good neighborliness.

It wasn't until a half hour later that Haig was able to rush into the White House, grab the first telephone in sight and set in motion the chain of decisions that would lead later that day to a U.S. condemnation of the raid and, two days later, an order to suspend delivery of four F16 fighter-bombers that had been scheduled to go to Israel Friday.

Even then, Reagan, and Haig weren't able to give their full attention to the consequences of Israel's unprecedented and defiant act. Together with almost everyone else in the top layer of U.S. officialdom, they had to go to Camp David for a day and night of talks with their Mexican counterparts.

And it wasn't until 5 p.m. Tuesday that the senior members of the national security apparatus were able to sit down at the White House for a full-scale meeting about how to respond to the situation.

That sequence could serve as a metaphor for what many U.S. officials privately see as the principal lesson to be drawn from the Israeli raid and its still unfolding aftermath.

It was a sharp reminder that, in the arena of international relations, events invariably get in the way of plans and priorities and timetables and, as a results, foreign policy often is reduced to the necessity of reacting to events, even when it means going against the grain of the most elaborately planned policies.

Reagan and Haig have talked a lot about bold new policies that will bring an assertive tone of American leadership to world affairs. But, in last week's Mideast crisis, the administration, instead of leading events, wound up being manipulated by Israeli Prime Minister Menachem Begin into taking actions and positions without haveing any real control over what it was doing.

As one official involved in the process later conceded: "It didn't really matter whether we made a decision immediately or put it off until after Lopez Portillo was on his way home. There really wasn't even much of a decision-making process involved. We had to react, but given the realities of the situation, there was only one option available to us. You could almost say the whole thing was predestined for the time the Israeli planes took off."

What he meant was that the administration, despite its dismay and anger at the Israeli action, was precluded by the U.S. commitment to Israel's survival from expressing its disapproval with anything more than a symbolic wrist slap.

That, in turn, has created a number of potentially vexing new problems with which the administration soon may have to grapple in the Middle East and elsewhere. They include:

A possibly irreversible setback to Haig's hopes of convincing the more moderate Arab states that the greatest threat to the Middle East comes from the Soviet Union and enlisting them, together with Egypt and Israel, in a "strategic consensus" to counter this danger. The blow administered to Arab world pride by the Israeli raid seems certain, at least for immediate future, to reinforce the Arab's conviction that Israel is their paramount enemy.

Increased difficulties for U.S. efforts to convince such key Mideast states as Saudi Arabia and Jordan that the Reagan administration is pursuing an even-handed policy in the region and will use its influence to restrain Israel and make it more cooperative in resolving such thorny issues as the Arab-Israeli conflict and the Palestinian problem.

Concern that the precedent set by Israel's preemptive strike might embolden other governments to attack neighboring countries they fear might be producing nuclear weapons. In the wake of the Israeli attack, there has been a rash of speculation about the impact on other nuclear-proliferation disputes such as that between India and Pakistan.

Increased domestic and external pressures on Egyptian President Anwar Sadat, who was embarrassed greatly by the Israeli move and who now will find it even more difficult to end the isolation from the rest of the Arab world caused by his rapprochement with Begin.

The likelihood that the relatively easy-going U.S. reaction to the raid will help Begin rebuild his standing with Israeli voters and retain power when his country elects a new government later this month.

This last point is especially ironic because the Reagan administration came into office convinced that Begin, whom it regards as too inflexible and reckless, would be an almost certain loser in the Israeli elections.

As a result, the priority scale established by the administration called for holding Begin at arm's length until he could be replaced by someone more responsive to U.S. views, while using the interim period to push Haig's concerns about Soviet encroachment on the Saudis and other Arab governments.

However, that approach failed to take into account Begin's flair for repreatedly manipulating events in ways that have diverted the administration from pursuing its goals while rebuilding his domestic political standing.

He did it first by putting the administration on the defensive with Congress over plans to sell sophisticated radar planes to Saudia Arabia, then by engaging in a confrontation with Syria over Lebanon that forced Washington to boost his stock by sending a special envoy to mediate the conflict, and now by pulling off the raid against Iraq in a way that enabled Israel to escape with only minimal and symbolic punishment from the United States.

A review of what went on in administration policy-making circles after the raid confirms, as one State Department offical ruefully put it, that "Begin is a master at doing things in a way that leaves you no option other than to go along with him."

U.S. officials agree that beginning last Sunday afternoon, when the U.S. Embassy in Tel Aviv sent secret word of the raid, the dominant feeling in senior administration circles was that, like it or not, there was little the United States could do to punich Israel that wouldn't have counterproductive effects.

There was some sentiment within the Defense Department for taking a more punitive stance by making the suspension of U.S. aid more broadgauged. But, although Begin later was to make much of this in an angry public attack on Defense Secretary Caspar W. Weinberger, the officials said this hard-line approach was rejected quickly.

It was opposed by Haig, who argued against getting into what could become a point-of-no-return confrontation, and White House counselor Edwin Meese III, who was concerned about an adverse political reaction from Amercican-Jewish voters.

By the time of the National Security Council meeting late Tuesday afternoon, the sources said, it had been decided tacitly that the punishment would not go beyond a temporary holdup a delivery of the four F16s and that most discussion then was centered on how to make the situation appear palatable to the Arab world.

To that end, the administration hopes that the ongoing debate in the U.N. Security Council will enable the United States support a resoultion condemning Israel, but without imposing any sanctions.

On a longer range basis, the administration also hopes that the need to make a compensatory gesture toward the Arabs can be used as an argument to help push the Saudi plane sale past its opponents in Congress.

Still, U.S. officials concede that these are, at best, damage-control measures and that the administration seems destined to come out of the situation trying to get its evolving long-haul Mideast policy back on track in the face of Heightened Arab suspicions and the likelihood of having to deal with a newly re-elected Menachem Begin.