THE REAGAN administration has made a sensible approach to the onslaught by guerrillas of the left in El Salvador. They had launched an offensive explicitly to test Mr. Reagan.He responded well, by reaffirming readiness to provide military aid to the civilian-military junta, and by moving to check the flow of foreign communist military support to the guerrillas. In respect to Nicaragua, through which arms and men move to El Salvador, the president is threatening to cut off American economic aid. Officially, Nicaragua denies assisting the guerrillas. Interestingly, however, in private it has asked for time to inspect the American evidence. It has closed down a clandestine radio station broadcasting into El Salvador. Is Nicaragua getting the message?

Regrettably, guerrilla violence, whether sustained from inside or outside, is only part of the security problem in El Salvador.There is no real argument that most of the estimated 10,000 political fatalities in 1980 were victims of government forces or irregulars associated with them. Any success in thinning the support coming in from Nicaragua would be politically advantageous to the junta and the United States, but by itself it might not bring much extra security to many citizens of El Salvador. That can happen only if the junta brings its armed forces and their unofficial comrades under control. That will take Mr. Reagan's vigorous encouragement, which so far has not been forthcoming.

As frightful as the security situation is in El Salvador, no serious movement toward an enduring solution can be expected without addressing the deep underlying economic and social inequities -- in a word, reform. Here there is a mojor dilemma. The land and related reforms spurred by Jimmy Carter are flawed, it is widely agreed, in both conception and execution. If they had added to uncertainty, however, they have helped hundreds of thousands of Salvadorans determine never to go back to the old life.

The Reagan administration, some of its previous statements notwithstanding, does not seem hostile to the idea of reform. But it is taking its time in framing a specific approach. The president has reason not to embrace the whole Carter-period package. But he must make a firm and quick choice of the worthiest elements in it. Whatever gains may be made in security will be of little avail if they are not underpinned by gains in economic and social justice. Support of reform, moreover, provides the only basis on which the United States can hope to meet the diplomatic necessity of having a policy at least somewhat consonant with that of Mexico.

These days El Salvador is seen by many on the left as another Vietnam, another American tragedy in the making. Many on the right also see El Salvador as another Vietnam, which to them evokes the specter of communism on the march. But El Salvador is not Vietnam, as either the left or the right defines it. El Salvador is itself, a tormented country with an old love-hate relationship with the United States. For the Reagan administration, dealing with it will be hard enough without having misleading metaphors thrown into the mix.