IN OPPOSING arms sales to Saudi Arabia, Congress seems to forget that there is a real war going on in the Persian Gulf -- between Iran and Iraq -- that will shape the political balance in the region for a generation.

Rather than paying attention to this actual war, Congress prefers to focus on the possibility of a Saudi role in the Arab-Israeli conflict. This seems odd, given the fact that Saudi Arabia has never played even a marginal role in the Arab-Israeli military balance. Nor is there any prospect that it could in the foreseeable future.

What happens when the Senate votes this week on the latest Saudi arms package probably matters less than the underlying trend that has developed over the last several years: the U.S.- Saudi defense relationship, on which both sides had placed great hopes, is slowly unravelling.

The United States probably won't pay any real price until the 1990s. Current energy projections suggest that Gulf oil won't become critical again for nearly a decade. By that time, however, there may be little left of the American-Saudi security relationship that both sides labored for more than 40 years to create.

The demise of the U.S.-Saudi relationship is already having important political consequences in the kingdom. In Riyadh there is a nearly universal feeling that the relationship with the U.S. hasn't worked out as hoped. And there is a search for alternatives.

Those who urged the kingdom to put its trust in the U.S. are now seen as having led the country down the garden path. Pro-American voices are hard to find, and those who have been closely associated with the U.S. are being ignored or excluded from the Saudi inner councils. Other Saudis, who have persistently argued for an accomodation with Iran, Syria and even Moscow, increasingly seem to have the ear of the Saudi leadership. They are saying: We told you so.

Often unspoken but always present in Saudi defense planning was the expectation that the United States would provide a military umbrella for the kingdom and its oil fields. American assurances to the kingdom go back to President Franklin D. Roosevelt; these assurances have been repeated by each president since, with special emphasis after the oil crises of the 1970s.

The United States helped build nearly every component of the Saudi defense system. Americans advised on both equipment acquisition and defense infrastructural development. They provided vital logistic support as well as training to the Saudi armed forces. The deployment of airborne early warning aircraft (AWACs) to protect key installations in the Gulf and the later decision to sell such assets to Saudi Arabia were essentially American decisions, reflecting both important U.S. interests and assessments of where the U.S. thought Saudi and Gulf defense should move.

The U.S.-Saudi relationship has been strained in recent years by frustrations on both sides. Saudis looked to America to resolve the Palestinian problem; Americans looked to the Saudis to stand decisively against Syria, Libya and the other radical Arabs. Neither side could meet these unrealistic expectations.

The tensions have increased with the American war against terrorism, which to the Saudis has the appearance of an anti-Arab campaign. Gulf leaders generally abhor Qaddafi, his politics and his proximity to Moscow. But they are deeply disturbed by American military attacks against Libya. In Arab eyes, the confrontation takes on the image of the West against the Arabs, Christians against Moslems, the strong against the weak.

These problems, although serious, have been manageable for the Saudis, if it wasn't for the humiliation inflicted by the recurring congresssional debates over arms sales.

In 1985, the Reagan administration and Saudi Arabia negotiated additional arms sales designed to enhance the kingdom's ability to defend itself and its neighbors. Some of the items had long been required to meet deficiencies; others were simply additional supplies of equipment already in the Saudi inventory, but in small quantities.

When it became clear last year that the sale was in serious trouble, it was postponed until this year. In the meantime, the most important items -- additional F15s, for example, that would enable the Royal Saudi Air Force to deploy and rotate their planes more rationally -- were deleted from the "package."

Now even the smaller "package" for Saudi Arabia, a proposal containing nothing beyond what is already in the Saudi inventory, appears to be in jeopardy. More striking yet, some are suggesting Congress should prevent the transfer to Saudi control of the AWACS sold to Saudi Arabia in 1981. If the administration succeeds in securing enough votes this week to prevent an override of a presidential veto of legislation rejecting the sale, can it muster enough votes for a second political battle later in an election year? Will it choose to do so for Saudi Arabia?

Both parties, the United States and Saudi Arabia, are clearly being pulled out of their defense relationship by forces they are powerless to combat. As the Arab-Israeli conflict becomes increasingly polarized, the U.S. is pulled closer to Israel and the Saudis closer to the center of gravity in Arab politics. It is an ineluctable process. But what is the effect likely to be?

There can be no doubt that the deterioration of U.S.-Saudi relations will have a serious adverse impact on Gulf security. The lack of firm and credible U.S. military support wipes away an entire dimension of Saudi and Gulf defense.

Some defense planners may argue that the arms-sale problem won't alter the basic American guarantee of Saudi security. Washington would still intervene, the argument runs, if the Soviets should ever attack the Gulf. But this analysis ignores the policy adjustments the Saudis are likely to make in the meantime because of the clear unreliability of American support. Indeed, the erosion of the U.S. protective presence may increase pressures on the Gulf states to normalize relations with the Soviet Union, which in turn will make it even harder for the Gulf states to buy American military hardware.

The Saudi defense structure that was built up over the last several decades will inevitably suffer. U.S.-Saudi training programs will be more limited. There will be less joint contingency planning. The question of "interoperability" with U.S. forces will become increasingly irrelevant. Equipment standardization among the Gulf states, always limited, may increase, but it will be dictated by political availability rather than technical suitability.

The trends in Gulf security, once very promising, now appear quite ominous. The underpinning of the emerging security system in the Gulf was the long and cooperative U.S.-Saudi relationship. Without this anchor, security can only be a hit-or-miss affair.