This two-day, 22-nation summit of rich and poor nations may best be remembered as the opening salvo in an international news briefing war, and if the United States came with the most ammunition and experience, it leaves today with the knowledge that it no longer has a corner on the noncommunist propaganda market.

Officials from industrialized and developing countries appeared to have finally caught on to what the United States has known for years -- that the press is ever-hungry for information, and will reflect the views of whomever gets to it first with the most news.

For the first time in an international forum, the Third World, and even normally reticent U.S. Western allies, matched the Americans briefing for briefing and managed to get across their contrary views on the two key summit issues -- the need to hold global negotiations on how to distribute the world's riches, and the establishment of a separate fund in the World Bank to finance Third World energy needs.

Thus, although U.S. spokesmen argued that the situation in private was different, in public it appeared increasingly that the United States was left holding the public bag in opposing what 21 other governments here appeared to favor.

During his on-the-record press conference Friday, Nigerian Foreign Minister Ishaya Audu, stated what seemed to be the prevailing view in Cancun: "If one country out of 22 appears to be taking another position, perhaps that country ought to look inside and say to itself, 'Can I be the only one right, and the other 21 all wrong?' "

But while words were flung with abandon across the ever-busy briefing halls, substance and meaning often appeared to get lost in jargon -- apparently intentionally in many cases. Everybody ended up in favor of "global negotiations," but no one seemed able to define what they were beyond some sort of vague U.N. talks about financial troubles.

In the end, the Americans have appeared to have won the battle by changing the name of the briefing game from information to obfuscation. Not that the other delegations, playing a catch-up game, did not do the same. The Americans just did it better, and delegates left Cancun divided on whether Reagan had "really" agreed to "global negotiations" or had merely agreed not to disagree with the concept.

As an obviously weary Canadian Prime Minister Pierre Elliott Trudeau put it last night, in a paraphrase of the final summit statement, the matter of global negotiations "has been left to some unspecified mutually agreed procedure at the United Nations . . . . If you said that begged the question, I would agree with you."

But there was no dearth of attempts at explanation from official spokesmen.

Over the two days of the conference, with an extra half-day tacked on at either end, Mexico briefed, Japan briefed, and Algeria briefed. Austria and Canada briefed several times. Venezuela briefed and Mexico -- whose officials said that as cochairs of the conference, representing the 14 nations of the South, they had no intention of letting the North control the flow of information -- briefed again.

China and Saudi Arabia were among the few who did not get in on the briefing spree. One Chinese journalist explained, "It's not part of our tradition."

Britain, West Germany and France came here feeling that they had lost out in past large-scale international meetings to the U.S. standard of three daily briefings, private press gatherings with high-level officials and rapidly reproduced documents.

In particular, they recalled the summit of the seven industrialized powers in Ottawa in July, where to some degree they were also at odds with U.S. policy and public description of what went on, yet found even their own national newspapers reflecting the overpowering American view of the closed-door proceedings.

This time, they coordinated their strategy at a meeting of Common Market foreign ministers in London last week, where a principal subject of discussion, a West German official confided, was how to operate at Cancun.

The dust scarcely had settled from the touchdown of Prime Minister Margaret Thatcher's plane Wednesday evening when her foreign secretary, Lord Carrington, called the international press together here for an on-the-record talk.

"I thought it might be useful," Carrington began pointedly, "if I took the opportunity just to say a few words about the British approach to this conference, so that those of you who are interested may know the ideas that we have and the British attitude to it."

Britain, Carrington said, supported "the creation of an energy affiliate of the World Bank," and "global negotiations in whatever form may be useful."

To the ear trained in the jargon of world economic issues, these were key phrases. They touched on two issues sure to be discussed here -- the establishment of a separate World Bank fund to finance rising oil costs for the Third World; and allowing the U.N. General Assembly, where the Third World has an absolute majority, to be a forum for full dialogue on changing the Western dominated international financial structure -- that the industrialized West was thought jointly to oppose.

Carrington's statement, a U.S. official grumbled, was "not helpful."

During the next two days, Britain matched the United States briefing for briefing and transcript for transcript, as Thatcher's spokesmen traveled several times each day from the off-limits Cancun Sheraton, where the summit meetings were being held, to the International Press Center.

There was informed speculation that the British were aiming not only at the United States but at their most traditional rival, the French, whose comparatively low profile here was thought to be a fear of overkill following French President Francois Mitterrand's own blitz of speeches and interviews on a U.S.-Mexican tour just before Cancun.

"They're happy anytime they can stick it to the Quai," a Briton noted in reference to his government's occasional irritation at the French foreign ministry, whose promotion of Third World issues over the last several months is thought to have overtaken Carrington's residual glow from the successful l979 Zimbabwe independence accord.

As for the West Germans, reporters from key Western media outlets were telephoned early on Thursday, the conference's opening day and offered a meeting, repeated on Friday, with a well informed "German official." That official said that his government also supported both the energy affiliate and global negotiations. An advance text of Foreign Minister Hans-Dietrich Genscher's prepared remarks to the conference was read, in time for early deadlines.

At times, the world outside the well-guarded Sheraton seemed more like an international trade fair than a summit of world leaders. Numerous national delegations filled press room tables with handouts ranging from Philippine President Ferdinand Marcos' biography to Nigeria's pamphlet, "Meet Mister President" on Shehu Shagari to Mexican Cancun summit ashtrays and beautifully photographed, slickly printed publications by France and Japan concerning their good relations with the Third World.

Part of the problem was that the summit meetings themselves were closed and the delegations managed to keep all but the merest trickle of inside conversations from leaking out. The few emerging tidbits spread through Cancun like wildfire--a confrontation between Reagan and Tanzania President Julius Nybrere over free enterprise versus agriculture subsidies; co-chairman Trudeau's repeated remonstrances to heads of state who kept the floor too long; Saudi Arabia's surprising acquiesence to some form of energy affiliate; Reagan's well received praise of India's "green revolution."

There was, to many observers, a remarkable reticence to criticize the United States, especially among Third World countries whose rhetorical stores normally abound with anti-U.S. slogans. Algeria, which Brazil's foreign minister had characterized as at the far extreme from the United States on the 22-nation scale of radicalism, said last night it was leaving the conference with a feeling of "great enthusiasm."

In the end, despite the seemingly endless verbiage and mountains of paper distributed to the press, few observers or even participants here seemed to have a clear idea of what had been decided. Only Reagan appeared to be sure and, in a final statement to the press released on arrival at Andrews Air Force Base, the U.S. president said the conference accepted most of his ideas.