Saudi Arabia, America's most important economic and poltical ally in the Persian Gulf region, appears to have reached a critical point in its already wavering attitude toward the United States and to be developing a "European option" to offset its traditional reliance on America.

As the Reagan administration weighs a Saudi request for additional equipment to "enhance" the firepower of its 60 American-built F15 fighter planes, the Saudis are keenly watching for signs that the United States recognizes a need to repair the badly frayed "special relationship" that has long existed between the two countries.

The atmosphere here is one of nervous anxiety as the kingdom awaits decisions in Washington that could be mementous for the future of Saudi-American relations in all fields from oil and arms to Middle East diplomacy and Persian Gulf Security.

Aside from wanting an early answer to their F15 request, however, the Saudis seem to be in no hurry to see the Reagan administration make up its mind on the overall Middle East policy. They are hoping a long and thorough review, particularly of the stalled Camp David peace process, will result in new directions making possible a continuing close association with Washington.

But the Saudi royal family has been seinding out strong signals over the past year of its serious intention to shift away from its primary dependency on the United States for security and arms if the new administration is not forthcoming. It has begun laying the groundwork for a viable "European option" as a hedge against a serious deterioration in its traditional American one.

At stake is not only billions of dollars in Saudi arms contracts but possibly similar amounts in related deals. Already, the kingdom has stunned the State Department and Pentagon by awarding a $3.4 billion contract to France for the next generation of naval ships and by asking West Germany to drop its ban on arms sales to regions of tensions and to sell it tanks and other weapons systems involving another $1 billion.

The Saudis are also reported to have asked Britain about the possibility of buying 200 Tornados -- an advanced fighter-bomber built by a British-Tialian-West German consortium -- at an estimated cost of $7.5 billion.

The F15 issue has gone far beyond a pure military matter so assume the proportions of what retiring U.S. Abassador John West calls a "litmus test" of the entire American attitude toward Saudi Arabia. "Because the Israelis oppose it and some other factors, it has become a cause celebre and a point of honor," he remarked in an interview.

One "other" factor, in the view of a non-American Western diplomat, is a natural process of "decolonization" in U.S. -Saudi ties. "You had a relationship which for the past 10 or 20years was almost colonial. They [the Saudis] have grown up now and realize they [can] get things elsewhere."

Another has been the impact of the Iranian-Iraqi war, which provided the Saudi royal family the proof of the legitimate military need for a better-equipped F15 aircraft as well as early warning (AWACS) radar planes. b

U.S. waffling over the F15 issue has infuriated at least part of the royal family and helped to generate pressure for the development of much stronger military ties to Europe. Defense Minister Prince Sultan is said to be particularly upset by the inconsistency of the past Carter administration, which asured him last summer through former secretary of defense Harold Brown that the enahancement items would be forthcoming only to have that assurence contradicted by President Carter himself a few days before the presidential elections.

The issue actually involves five diffrent items associated either with the aircraft itself or with the overall Saudi air defense capability. They include wing fuel tanks, air-to-air "head-on" missiles, bomb racks, transport planes for in-air refueling and the AWACS early warning planes.

"They are not going to buy a caponized version of the F15," remarked one American official.

"They are certainly going to finance the [French] Mirage 4000 and buy the Mirage 2000 if we don't give them F15s," said another informed source familar with Saudi military views. "They don't have that much invested [in the F15] they couldn't pull out."

While the Saudis have placed their order for the U.S. aircraft, they are not scheduled to actually take possession of the first of them in Saudi Arabia until next year, after completion of training on them in the United States. So far, they have paid only a portion of the $3.5 billion price.

U.S. thinking about Saudi intentions began shifting radically last spring when the kingdom signed what was initially a secret $3.4 billion agreement with France for ships and helicopters, cutting a new wide swath into a theretofore American preserve.

The order was placed even though the United States was having great difficulty in implementing its own $4.5 billion, 10-year Saudi naval expansion program involving the delivery of 19 ships and the training of sailors to man them because of a manpower shortage.

The royal family's decision to turn to France for the next generation of naval equipment was sufficiently controversial even within Saudi circles to provoke the resignation of the naval commander, according to several Western sources who believe, however, that other considerations may also have been involved.

The decision parallels developments in other areas where the Saudis are turning increasingly to France and now West Germany for assistance to balance, and possibly replace, their longstanding dependency on the United States.

These areas include the Army, where the French are training the Saudis on their AMX tanks, and the Air Force, where the kingdom has reportedly invested at least several hundred million dollars and perhaps closer to $1 billion on research and development of the French Mirage 4000, an advanced technological twin jet fighter.

Another field where the French are making inroads is internal security. A French advisory group is now serving in the Saudi Interior Ministry and the Saudis are said to be thinking of setting up the equivalent of highly mobile Campagnie Republicaine de Securite' in the French police force.

More recently, the Saudis are reported to have asked the West Germans to sell them 300 Leopard II tanks and other unspecified weapons systems in a deal being qualified in Bonn as initiating another "special relationship" between a Western nation and the world's giant oil exporter. The kingdom last year lent Bonn $2.3 billion to help cover its current accounts deficit and is buying an additional $2.75 billion in West German bonds this year, thereby expanding the two countries' financial interdependence as well.

Virtually every American official here and in Washington dealing with Saudi Arabia is convinced that the French and West German deals are partly, and possibly primiarily, political signals meant for Washington. But some cast the Saudis' decisions into a larger picture of a deliberate, if still unannounced, policy of loosening their ties in the military field with the United States as an assurance against what is viewed here as American subservience to Israeli dictates in Middle East arms matters.

"Israel as the arbiter of Saudi defense requirements is intolerable," reflected one American diplomat.

The Soviet invasion of Afghanistan and particularly the Iranian-Iraqi war have heightened the Saudi sense of vulnerability.

The fighting time from Iran to the big Saudi oil terminal at Ras Tanura on the gulf is just 14 minutes, and the existing Saudi ground radar system gives the Air Force just six minutes to scramble and intercept an incoming jet.

The SWACS, on the other hand, picks, up Iranian jets from the moment they leave their air bases, appreciably increasing the warning time in a situation where minutes can make a lot of difference, according to sources familiar with the performance of the aircraft.

Saudi and American sources say the war has also proven the need for all the so-called enhancement items, except perhaps the bomb racks intended for use in case of a war with Marxist South Yemen.

Both the wing fuel pods and tankers for refueling would enable the F15 to reach the gulf and remain in air for some time when operating from fields around Riyadh, where, according to these sources, they would have to be based for protection in case of a gulf things stand now, the F15 would remain extremely vulnerable to attack if stationed in dahran on the gulf. w

As for the AIM9L air-to-air missiles the saudis want, these can be fired head-on at an incoming plane rather than from behind and would again save invaluable minutes in situations where every one counts.