Two AWACS aircraft sent to Egypt last week underscored the extent to which the electronic watchdogs known as Airborne Warning and Control Systems are now supplanting old show-the-flag "gunboat diplomacy."

Simply by dispatching the two surveillance aircraft, the United States was able to project a dual-purpose symbol of its military and political presence into the uneasy region after the assassination of Egyptian President Anwar Sadat.

It is because the AWACS aircraft have acquired such exceptional significance as an international status symbol, of course, that the dispute is so intense in Congress over selling them to Saudi Arabia.

For more than three years now, in two administrations, AWACS aircraft increasingly have become a major instrument in the conduct of foreign policy.

They were sent to real or potential trouble spots at least five times during the Carter administration, and the Reagan administration made its first political use of the radar plane last Wednesday. Congress has generally welcomed the use of the aircraft for political impact abroad as an alternative to greater military involvement.

Sending in the AWACS as a way of injecting a dramatic American presense into an area of turmoil is quicker and simpler than dispatching warships or Marine landing teams, and U.S. officials hope it is less entangling as well.

But specialists in this form of diplomacy caution that there is no cost-free or risk-free way to use weapons for political purposes. Too frequent use of the tactic, some specialists warn, cannot only nullify the impact but also generate new pressures for American military involvement abroad.

The AWACS are modified Boeing 707 aircraft used as electronic command posts, with their large rotating domes adding mystique to their appearance. They are being used to "show the flag," to "buy time" in ambiguous situations that may require a display of force, to make or underscore warnings or threats or to demonstrate fidelity to a friendly nation--as well as to gather intelligence and, if necessary, to coordinate the use of firepower against an adversary. In the language of the specialists, the unarmed planes can be either "passive" or "active" defense systems.

Relatively few Americans are aware of the cumulative extent to which the Boeing AWACS have been employed for politically related purposes.

In March, 1979, two AWACS planes were sent to Saudi Arabia for five weeks during the warfare between North Yemen, Saudi Arabia's ally, and Soviet-supported South Yemen. The AWACS provided a double measure of physical and psychological reassurance for Saudi Arabia, in addition to the American jet aircraft, tanks and other weapons and training teams sent to North Yemen in its short-lived war.

Two more AWACS were dispatched to South Korea in October, 1979 for 60 days, after the assassination of President Park Chung Hee aroused apprehension about internal turmoil that communist North Korea might exploit.

In June, 1980, Defense Department sources said, another two AWACS were sent to Kadena Air Base, Okinawa, two to three months earlier than planned, for regional security. At that time, students in South Korea had launched violent demonstrations against President Park's successors. This pair of AWACS is now permanently based in Okinawa as the Asian-Pacific counterpart of two AWACS permanently based in Iceland for defense of the northern region of the North Atlantic Treaty Organization.

In September, 1980, four AWACS were ordered to Saudi Arabia for dual military-political security requirements arising from the Iran-Iraq war. These American-operated AWACS are still in Saudi Arabia. It was one of these aircraft, as the late Egyptian President Sadat disclosed last August, that the United States assigned for his personal security on a flight to Sudan.

In December, 1980, four AWACS were assigned to West Germany at a peak of alarm over a possible Soviet invasion of Poland, although officially this assignment was described as an air training exercise. Two of these aircraft were withdrawn last January and the remaining two in April. NATO has contracted for 18 of its own AWACS by mid-1985, and Boeing sources said two of these aircraft are now being prepared in West Germany.

The latest dispatch of AWACS--to Egypt last Wednesday--will, according to Secretary of State Alexander M. Haig Jr., "permit us to demonstrate our overall capability" for "the security of the area." This is a show of support for Egyptian President Hosni Mubarak, as well as a warning to Libya against military intervention in the region.

The U.S. Air Force now has 24 AWACS, 10 of them overseas, Pentagon spokesmen noted. The Air Force had planned to expand its fleet to 34, but President Reagan recently expressed a desire to add at least six more, for a total of at least 40. Those numbers are almost certain to increase the temptation for expanding the political use of AWACS.

The opportunities and dangers foreshadowed by this are illustrated in a study of post-World War II usage of American armed forces for political purposes, made for the Pentagon by the Brookings Institution in 1977. The study concluded that "land-based combat aircraft" had advantages over the deployment of ships or ground forces to influence the perceptions and behavior of other nations. In its public form, this analysis was published as "Force Without War, U.S. Armed Forces as a Political Instrument," by Barry M. Blechman and Stephen S. Kaplan. A companion study of comparable Soviet practice was later published by Kaplan.

In many instances, the Blechman-Kaplan review found, a "show of force" by the United States was valuable in "buying time" for a limited period, in situations of sudden change and high tension, to provide opportunity to formulate a new policy or to try to induce an adversary to change course. But they cautioned that the practice can be no substitute for lasting policies "tailored to the realities of politics abroad. . . ."

The use of armed force as a political instrument "does not come free," Blechman, now a senior associate of the Carnegie Endowment for International Peace, emphasized recently. It also carries the danger of "polarizing a situation," he said, and he finds that possibility especially hazardous when the United States currently is "making a lot of commitments tying our prestige to a lot of un-stable regimes." Therefore, he said, this process should be used "judiciously--not scattershot.