The Reagan administration has clearly chosen El Salvador as the best of the options available to send a signal to -- and even test -- the Soviet Union. In doing this so early in his presidency, Mr. Reagan is reversing the pattern that has developed in Soviet-American relations whenever the US presidency changes hands. That pattern, since the days of Nikita Khrushchev and John Kennedy, has had the Kremlin initiating the test to get the measure of a new and unknown American president.
In the eyes of the Reagan administration -- and of a growing number of other Americans -- the Soviets have used detente since its inception under the Nixon presidency as a smoke screen. Behind it, they have "got away" with expansive thrusts to establish footholds in Angola, Ethiopia, South Yemen and, most recently, Afghanistan.
Mr. Reagan's apparent aim is to say to the Kremlin at the outset of his presidency: "Thus far -- and no farther!"
But there are risks in the hardening of US policy on El Salvador. They include:
* Overemphasizing the Soviet and Cuban role in the insurgency in El Salvador and thereby ending up with a prescription for that country's woes and problems that is out of focus, ineffective, and even counterproductive.
* Encouraging the murderous right-wingers in El Salvador's armed forces and the country's supposedly 14 privileged families to believe that the US has reversed signals sent by the Carter administration and that they can forget political and social reform and push the political pendulum back to where it was before the ouster of dictator Carlos Humberto Romero in October 1979.
* Getting caught in a Central American quicksand, should US troops ever be introduced in a combat role in El Salvador. (Hence Mr. Reagan's reiteration that this will not be another Vietnam, offset -- perhaps understandably if the US stand is to be convincing in Moscow -- by adviser Edwin Meese's assertion that no option can be excluded.)
* Inviting murder squads from the extreme left to pick off Americans introduced into El Salvador under the new Reagan policy -- whether those Americans are in a combat role or not.
* Alienating a huge segment of Latin American opinnion, most importantly the government of Mexico, which is latently paranoid about "Yankee" strong-arm interference in Central and South America.
* Alienating to a lesser extent European opinion by confirming latent European fears about a roused US nationalism acting too impetuously and with too strong an arm -- particularly over a Central American mini-state like El Salvador. (Hence President Reagan's efforts to reassure in recent days the British prime minister and the French foreign minister, to be followed this week by a similar exercise with the West German foreign minister.)
There are offsetting arguments supporting the Reagan administration's apparent decision that if there is to be a test case with the Kremlin at the outset of the new presidency, El Salvador is the best choice. These include:
* The georgraphical location of El Salvador -- in Washington's own Caribbean backyard, to quote a cliche --which puts it indisputably in the internationally recognized American, not the Soviet, sphere of influence. Consequently Moscow is more likely to back away in the last resort, rather than risk nuclear confrontation with the US.
* The way in which El Salvadorhs location helps the US to cut off the Salvadoran insurgents from the source of the arms they have been getting through Soviet/Cuban channels or proxies. The only neighbor "friendly" to El Salvador's leftists is Nicaragua -- where the Sandinista government is reportedly having second thoughts about its earlier support of the Salvadoran guerrillas.
* The relative ease with which a fairly short coastline can be patrolled to hinder gunrunning by sea -- which explains the naval component in the team of US advisers being sent to El Salvador.
* The fact that the leftist guerrillas in El Salvador may already be on the defensive. First, the leftists' all-out offensive, just prior to Mr. Reagan's inauguration, was a failure. Second, there was no mass uprising of popular support for the leftists at the moment of their offensive. (There was mass support in neighboring Nicaragua for the leftist Sandinistas, who successfully ousted dictator Anastasio Somoza in mid-1979.)
* The likelihood that outside critics of the Reagan policy -- including those in Mexico and Western Europe --will applaud in the end if it succeeds. The criticisms admittedly reflect current doubts -- and thus may well be self-preserving insurance against US failure, rather like Saudi Arabia's criticism of the Camp David peacemaking process in the Middle East.