BRITAIN'S AGONY with Argentina has some remarkable parallels with the Suez crisis of a quarter century ago.

Then, as now, the whole British nation rallied behind its government with patriotic fervor as its immediate response to Gamal Abdel Nasser's unexpected nationalization of the Suez Canal on July 26, 1956.

The scenes of today are right out of the early period of the Suez crisis: Noisy threats echoing through Britain and much macho posturing and flexing of military might. Warships sailing from Portsmouth to the cheers of families and friends. Banners jauntily declaring from the ships' decks: "Watch out, Nasser, here we come."

Britain's prime minister, Anthony Eden, set the national tone by saying that "the Egyptian has his thumb on our windpipe. I'm not going to let him get away with it." The raucous London press seconded his bellicosity. The Daily Sketch urged the government to "prove that its legs are not completely palsied by getting up on them and raising hell." The Herald exhorted: "No more Hitlers."

Today, World War II memories are invoked again. Foreign Secretary Francis Pym declares that Britain "does not appease dictators," and The Times accuses Argentina of "naked aggression" not seen since the days of Hitler.

Then, the Labor and Liberal opposition joined the warlike chorus, declaring Nasser's nationalization deplorable (Liberal) and unjustifiable (Labor). Winston Churchill, who had retired only the year before, expressed the national mood most colorfully by declaring that "we can't have that malicious swine sitting across our communications."

Despite such enthusiastic early support, Eden soon saw it dwindle away in the passage of time -- and Margaret Thatcher could face the same enervating delays.

In Eden's case, the country had neither the landing equipment nor the troops with the special training required to undertake an immediate amphibious landing. His military chiefs needed six to eight weeks to gather the proper force. After that, stalling diplomatic moves by the United States delayed the operation until the end of October.

During that time, Eden's domestic support vanished. The opposition political parties started questioning whether indeed force was justified. Some of the major newspapers, initially hawkish, also developed second thoughts. Even members of Eden's own government began questioning the wisdom of an invasion. By the time his military operation against Egypt finally got underway, Eden found his country passionately divided.

In an age of television, when time and evens are telescoped, Mrs. Thatcher may also discover that the two weeks needed for the arrival of her ships off the Falklands could leave her with seriously eroded support. "

Another parallel with Suez is the role being played by Secretary of State Alexander Haig as he shuttles between London and Buenos Aires. One of Haig's predecessors, John Foster Dulles, played a similar role during the Suez crisis. He traveled repeatedly to London to try to pacify Eden. When that failed, he invented one diplomatic delaying action after another to prevent hostilities.

Dulles and President Eisenhower were motivated by a consideration that seems to be weighing on Ronald Reagan today. This boils down to maintaining some U.S. influence in the Third World by not automatically throwing U.S. support behind a former colonial power.

Yet Reagan's actions now are in a sense much less difficult than Ike's because of the simple fact that Egypt had acted entirely legally in 1956 while Argentina has not.

Such a course was an uncomfortable one for Ike, for he had no love of Nasser. Thus Eisenhower, though he admired the British and valued their friendship, felt he had to oppose British force on grounds of principle as well as international politics.

The difference in Britain's position now and during Suez is pointed up by the way London has used the United Nations. It had no case against Nasser in 1956 and at first avoided pushing the issue in the U.N. When it finally did go to the U.N., it was in an effort to sidetrack Washington's delaying tactics and get on with its invasion.

This time, of course, Britain went immediately to the Security Council and had Argentina branded an aggressor. That may not deter the use of force, but at least Britain enters the fray with a clean slate.

A parallel that has not occurred but which should be causing the greatest concern is the utterly unpredictable way in which the Suez crisis finally played itself out.

At the beginning, the crisis was a fairly clearcut dispute over an international waterway. Before it ended, France and Israel had secretly joined with Britain -- without Washington's knowledge -- in an attempt to topple Nasser. The Russians, who at the same time were brutally stamping out a rebellion in Hungary, issued veiled threats to launch missile attacks against London, Paris and Tel Aviv. In reaction, Ike alerted the Sixth Fleet to be ready for war.

Suddenly, out of a crisis that at first appeared fairly simple and manageable, the world was closer to thermonuclear war than it had ever been.

The affair ended as a disaster for Eden. Once considered one of the world's leading statesmen, he collapsed in ill health and soon was thrown out as prime minister. Britain never afterward could posture as a world colonial power. The days of glory were over, as The Times of London noted in its obituary of Eden when he died in 1977:

"He was the last prime minister to believe Britain was a great power and the first to confront a crisis which proved she was not."

Amid today's jingoistic cries and comic opera strutting, those convoluted events of a quarter century ago offer a cautionary warning. While Britain has right on its side this time, the reckless use of force has a way of turning seemingly clear issues into trash heaps for once glorious reputations