SALT II was the little engine that couldn't -- a treaty that took too long to negotiate, that lacked persuasive presidential sponsorship and that won too few friends to survive the buffeting winds of international crises.

The latest blow to the strategic arms limitation treaty's prospects -- yesterday's announcement that the Senate will postpone its debate at President Carter request -- probably was fatal. At the very least, it will require extraordinary changes in the international situation and in American politics to put any life back in the treaty.

So the single most important elements in the tense and tenuous Soviet-American attempt to achieve a measure of formal detente has slipped into limbo, and prospects for cooperation in any field are in doubt.

Derailing this treaty was not easy -- it took five years. The basic outlines of SALT II were agreed to at Vladivostok by Leonid Brezhnev and Gerald R. Ford in November 1974, and soon Brezhnev was predicting early signing of a formal agreement.

But almost at once, events began to conspire against a new treaty. In the winter of 1975, a Soviet-American trade agreement collasped over the issue of Jewish emigration. Then the United States introduced the idea that two new weapons systems not mentioned at Vladivostok should be covered in SALT II, a further complication. Finally the pressures created by Ronald Reagan's challenge to President Ford in the Republican primaries of 1976 led Ford to overrule Henry Kissinger and abandon the Salt negotiations during that election year. h

The Carter administration came into office with high but vague hopes for progress on SALT. But Kissinger's ghost bedeviled the new team. Apparently acting out of a determination to do things differently than Kissinger had, the new administration abandoned the Vladivostok formula. Instead Carter proposed "deep cuts" in strategic weapons in an entirely new "comprehensive" SALT. In the spring of 1977, the Soviets angrily rejected these ideas, and several months later the United States returned to the Vladivostok formula as the basis of negotiations.

According to sources intimately associated with President Carter, he and his inner circle now regard the 1977 "comprehensive proposal" as a mistake.

It would have been much wiser, the Carter group now feels, to have moved quickly to put the Vladivostok agreements into treaty form and get them before the Senate. These sources speculate that the courses of the Carter administration as well as the SALT negotiations would have been much different if a Soviet treaty could have been approved before the divisive Panama Canal treaties reached the Senate.

In the fall of 1977, and article in The New York Times laid out the basic elements of the SALT II accord then being negotiated. With the essential details made public (they changed little in subsequent months), the debate began at once -- nearly two years before the Senate's first hearings on the treaty.

For many months, the debate was essentially one-sided. Skeptics sniped at the treaty from many directions. Pual Nitze, a former Pentagon official with keen mind and boundless energy, began a one-man crusade to transform or reject SALT II; it proved extremely effective. Nitze's arguments that the treaty gave unwarranted advantages to the Soviets by permitting them to maintain larger rockets capable of carrying more destructive bombs put the White House on the defensive. For a long time the Carter administration failed to enunciate a clear rationale for the treaty, rarely getting beyond pained ripostes to critics' accusations.

Beginning in late 1977, members of the Senate friendly to the treaty complained that Carter himself was missing his chance to set the terms for the SALT debate and dominate it. In response, the administration said it could not begin defending a treaty before it was signed, an argument that missed the essential point: The best reasons for supporting SALT II lay not in its detailed provisions but in the limited stability it offered, and in the dangers that awaited a world ungoverned by such a treaty.

The administration eventually settled on those arguments, but not until long after the critics' concerns were dominating the public debate.

Repeatedly, the Carter administration's hopes for a speedy conclusion of the negotiations were dashed. In October 1977, Carter said publicly that a treaty might be signed "in a few weeks." In December 1978, the administration was all but certain that final agreement was at hand. But the Soviet went into a stall, apparently to protest the American decision -- announced that same month -- to normalize relations with China.

If negotiations were delayed, though, developments in the world of armaments proceeded apace. What the experts sometimes call "the strategic environment" underwent a succession of changes between 1974 and 1979, each of them complicating the SALT process.

The most important change was brought on by the Soviet's enormous program of building and deploying new missiles and warheads. As the '70s progressed, the Soviets consistently moved faster and farther than American experts had predicted to improve their rocket forces.

In the process the Soviets violated one of the taboos of the arms race, in the U.S. view. The Russians moved so fast that, by the mid-'80s, they will have a theoretical "first strike" capability to wipe out America's land-based (but not its submarine-based) rockets. Any hint of first-strike capability was long considered a destabilizing, dangerous step in the United States, on grounds that the country against which it was aimed would get an itchy trigger-finger in any future crisis.

The accelerated development of American cruise missiles -- plane-like, unmanned, capable of unprecedented accuracy against which the Soviets have no defense -- added further complications. So did an effective new Soviet bomber called the Backfire. And so did the American proposal for a mobile, land-based missile system called MX.

Finally, the North Atlantic Treaty Organization allies -- responding to an unanticipated buildup of Soviet medium-range, mobile missiles aimed at Europe -- have decided to deploy new American missiles on European territory. Many of these missiles will be in Germany, Russia's traditional preoccupation in Europe. From a Soviet point of view, placing new missiles in Germany capable of striking the Soviet homeland would violate another old taboo in the Soviet-American competition.

Despite the delays and complications, though, SALT II's chances looked reasonably good as recently as August. July hearings on the treaty went well for the administration, which by then had developed a strong case that SALT was a marginally useful step, and that rejecting SALT would be dangerous. The most effective witnesses were the Joint Chiefs of Staff, who said that SALT II alone wouldn't solve America's strategic problems, but that rejecting the treaty would aggravate them.

This was a minimal defense of the treaty, one that might have produced a scant two-third Senate majority during the fall. But new strains intervened.

A mishandled "crisis" over a Soviet combat force in Cuba undid the summer momentum that had build up behind the treaty. The taking of hostages in Iran precluded the opening of formal debate in the fall.

Coming during an election campaign, the Soviet invasion of Afghanistan is likely to delay any action on the treaty until 1981.

The authors of SALT II insist that the treaty was written to last, and could still be brought into force next year or even later.Both superpowers would have to show unprecedented restraint to keep strategic arms control alive without a SALT agreement in force, however, and in an atmosphere of renewed Soviet-American tension, restraint may be difficult.

One stubborn optimist in the Carter administration wondered yesterday what might happen if Soviet forces are compelled to withdraw from Afghanistan, the hostages are freed from Iran, and President Carter decimates Sen. Edward M. Kennedy (D-Mass.) in the presidential primaries. d"If all that happened, we could get Salt through this year!" the source predicted.