What ever happened to "the most serious threat to world peace since the second world war?"

That was the way President Carter, in his State of the Union message to Congress last January, described the Soviet invasion of Afghanistan.

In that same speech, the president pledged the United States to defend the Persian Gulf against any outside force, meaning the Soviet Union. A buildup of U.S. forces in the region -- already under way because of the Iran hostage crisis -- was speeded up. Suddenly, there was talk of war in the air. Presidential emissary Clark Clifford, during a trip to New Delhi, warned gravely that the Russians must know "that if part of their plan is to move toward the Persian Gulf, that means war."

In recent weeks and months, however, Afghanistan seems to have become the forgotten war, at least in this country. In 90 minutes of debate last Tuesday, neither Carter nor Republican challenger Ronald Reagan mentioned it.

Since the Soviet invasion and occupation began 10 months ago, more than one million Afghan refugees have fled the fighting and their homeland. Afghanistan's representative to the United Nations Economic, Scientific and Cultural Organization (UNESCO) startled delegates to a Belgrade meeting last week by denouncing the Soviet actions and then defecting to the West, claiming later that more than a million other Afghans have been killed, a figure that cannot be verified here and which U.S. officials believe is exaggerated.

Nevertheless, the war has faded from the headlines and from administration rhetoric, overshadowed by the election campaign, the new drama surrounding possible release of the U.S. hostages, and the subsequent war between Iran and Iraq.

White House talk of confrontation may also have subsided intentionally as the campaign entered its crucial months and as Carter forces relied heavily on a campaign tactic that attempted to portray Reagan as the candidate who might lead the country into war.

Similarly, the administration chose to make arms control with Moscow another major campaign theme, something that required the White House to separate Afghanistan from the pledge to try to get the strategic arms limitation treaty (SALT II) with Moscow approved in the Senate, even though it was the White House that asked that the treaty be pulled from the Senate floor after the Soviet invasion.

Behind the scenes, Afghanistan has also been overshadowed by a potentially more explosive situation in Poland. At high levels throughout the Carter administraion, officials expressed the view that the continuing clash between the Polish labor movement and Communist Party authority, taking place against the background of a seriously troubled Polish economic situation, could lead to Soviet military intervention -- as a last resort -- in the year ahead. l

Later this month, the U.N. General Assembly in New York is expected to revive debate on Afghanistan, and the United States will once again lend its backing to a resolution first offered last January by 34 Moslem nations calling for the immediate and unconditional withdrawal of all Soviet troops.

The Soviets, however, clearly seem to be settling in for both a long fight and a long stay in Afghanistan, and interviews with a number of administration officials here indicate there are now new U.S. strategic shaping up to try and get them out. Rather, the major effort seems to be aimed at trying to preserve the existing strategy of grain, technology and certain other trade restrictions from being eroded by pressures in this country and especially from allies in Western Europe and Japan who have only reluctantly joined in some of the curbs.

The problem facing the administration is that the war in Afghanistan is far away and the public seems to be losing interest in it. American press coverage on the scene is limited to the relatively rare occasions when a reporter slips in and out of the country despite an Afghan government ban.

Many administration officials also now privately acknowledge that the president's description of the invasion as the gravest threat since World War II was probably a bit overdrawn. Many European leaders at the time also questioned the president's assessment, arguing that the Soviet invasion of neighboring Afghanistan was most likely aimed at heading off the overthrow of the pro-Moscow regime in Kabul by a growing and potentially volatile, anti-Soviet, Moslem-led insurrection.

Thus, with little ability to keep the war in front of the public here, and with many allies skeptical from the start, there is growing concern that it will become harder and harder for the administration to keep both this country and the allies in line on the sanctions.

Yet, as one administration official puts it, even if the Soviets went into Afghanistan because they saw it as a threat along their border rather than as a steppingstone to the Persian Gulf, "the fact is that they are there. They have penetrated a buffer region. They are physically nearer the Gulf with 85,000 troops. And if they come out of this with no skin off their noses, they may indeed view this as a precedent for future actions. It would be very tempting."

Whether a decrease in outside interest and support will have an effect on the rebels fighting the Soviets in Afghanistan is another problem that will grow as the war drags on. Will the rebels at some point conclude that further resistance is fruitless? Though U.S. officials have frequently given estimates of Soviet casualties in the fighting, it has been impossible to get an assessment of rebel casualties. Most officials, however, believe the rebel casualties are relatively light and that they can fight on for some time.

On the other hand, a major source of rebel arms -- regular Afghan army units that have defected to the rebel side -- is drying up as that army shrinks. The ability of the rebels to hold out, therefore, is going to depend increasingly on the ability to get more and better weapons from the outside, an extremely touchy subject.

It is known that some small arms and antitank weapons are being slipped into Afghanistan by the United States, Egypt and China. But more and bigger arms may be harder to slip in without dragging Pakistan further into the war because virtually all such arms go in across the Pakistani border.