More than a half dozen times over the past four years, the United States has used what might be called ''AWACS diplomacy.'' In most cases, the big airborne warning and control system (AWACS) radar planes have been deployed with the twofold purpose of reassuring friends and warning adversaries.
In the latest dispatch of the surveillance planes, however, the purpose was threefold, making it a more complicated maneuver than on previous occasions. According to US officials, the planes were sent close to the central African nation of Chad in order:
(1) To warn Libya's leader, Col. Muammar Qaddafi, away from supporting further attacks inside Chad.
(2) To reassure the Chadian regime of Hissene Habre of American support.
(3) And to encourage - some would say exert pressure on - the French to do more to support the Habre government and army.
The final verdict is not yet in. But Qaddafi has clearly continued to do more than the Americans would have liked, and, until recently at least, the French have clearly done less.
US defense specialists say that the two AWACS planes now based in the Sudan, situated directly east of Chad, would be capable of coordinating French fighter plane attacks against Libyan jets and armor should the Libyans continue to participate directly in the fighting in Chad. Chad's President Habre has called openly for French air support. The French have dispatched military equipment as well as a contingent of French paratroopers on what is described as a training mission. But they have thus far decided against sending fighter planes into the war.
In an unusually candid comment in a news conference last week, President Reagan acknowledged that the US had looked to France to intervene in Chad with its air power.
''Frankly, we had believed at first that there was going to be some aerial activity there,'' said Mr. Reagan, in a remark which astonished professional diplomats, who are used to proceeding cautiously in any public statements about the sensitive French.
In the same news conference, Reagan, in effect, ruled out American intervention on the ground in Chad and stated that the African nation is in France's ''sphere of influence.''
While the AWACS is largely viewed as a defensive system, it is capable of guiding attacking planes. A Defense Department official says sending the radar planes amounts to a more ''subtle show of military capability'' than sending warships.
The AWACS also has the advantage of usually being able to get to the scene of a conflict faster than a warship. As the magazine US News & World Report said recently, the AWACS is a ''high-technology version of gunboat diplomacy.''
The radar planes were first deployed in a crisis by President Carter on March 8, 1979, when he sent two of them to Saudi Arabia to monitor fighting between Marxist-led South Yemen and North Yemen. The AWACS planes reached Saudi Arabia March 9, almost a week before the aircraft carrier USS Constellation and other ships could reach the region from the Pacific.
''The beauty of the AWACS is that you can put it anywhere in the world within 24 hours,'' says a US Air Force spokesman.
In October 1979, following the assassination of South Korea's President Park Chung Hee, Mr. Carter sent two AWACS planes to South Korea. Their deployment was meant to warn North Korea not to take advantage of the uncertain situation. AWACS planes were also placed on alert in the region following internal unrest in Korea in 1980.
Four AWACS were sent to Saudi Arabia at the outbreak of the Iran-Iraq war. Two were sent to West Germany at a point when it was feared the Soviet Union might invade Poland.
In October 1981, Reagan sent AWACS planes to Egypt to show support for the country's new leadership following the assassination of President Anwar Sadat.
In February 1983, four AWACS were sent to Egypt with the announced purpose of participating in training exercises. But many observers concluded that the planes were there for a second purpose: to warn Libya's Col. Qaddafi of possible American retaliation should he threaten Sudan.
In each of these situations, too many other factors were involved for the effectiveness of the AWACS to be measured with any certainty. What is clear is that Egypt and Sudan have asked for the US to bring the planes back. And the AWACS have yet to be involved in a situation where a government friendly to the US was toppled.