The Reagan administration has been pointing with pride to progress in El Salvador: more democracy, less violence, a shrinking insurgency. Lt. Comdr. Albert A. Schaufelberger III, the first American serviceman to die in El Salvador two years ago, would have been reading the same evidence -- with alarm.

He would have been shocked by the brutal slaying of four off-duty U.S. Marines and nine civilians, including two Americans, in a sidewalk caf,e in San Salvador last week. But he would not have been in the least surprised. He would have seen it as logical, tactical turn to urban warfare against Americans in a guerrilla war when American support is turning the tide against the insurgency in the countryside.

We know this not as a voice from the grave but from an extraordinary interview with The New York Times, a few days before Schaufelberger was murdered on the streets of San Salvador. "They haven't targeted Americans because things are going so well," he told Lydia Chavez. But "if Pesident Reagan is successful" in bolstering the government with U.S. aid, the rebels "are going to get nasty."

Trained in counter-insurgency, Schaufelberger had no illusions about the nature of the struggle -- or the enemy. He knew the difference between nastiness for a particular purpose and "senseless terrorism" -- the White House way of connecting last week's killings in San Salvador with the bombing of the airport in Frankfurt, West Germany, and the hostage- taking from TWA's hijacked flight 847. It all added up to "further evidence" that "the war which terrorists are waging is not only directed against the United States (but) against all civilized society," the White House said.

The trouble with talking about "terrorist" problems in such epic, indiscriminate terms -- as the foot-soldiers in the world's increasingly shadowy warfare well know -- is that it leads to talking about solutions in the same simplistic way. Consider last week's presidential statement. "This cannot continue," it said, without defining "this." "We must act," the statement went on, adding: "Our limits have been reached." When White House spokesman Larry Speakes was asked what that meant, he said: "It means that we are now drawing the line."

What that line-drawing apparently means for now is more military help for the Duarte government in El Salvador. Meanwhile, Vice President Bush will take advantage of his trip to Europe this week to rally some sort of concerted international counter-terrorist program. As soon as he gets back he will set up a U.S. government-wide task force to help the president decide what to do.

You would think that the Reagan administration had not been witness for 41/2 years to a steady increase in terrorist acts, to the war in Lebanon, to an alarming spate of terrorist attacks against NATO installations (more than 80 incidents in the last year). You might even suspect that the administration is wracked by internal differences -- and you would be right, if the public record means anything.

Secretary of State Shultz tells us, "Experience has taught us over the years that one of the best deterrents to terrorism is the certainty that swift and sure measures will be taken against those who engage in it." Experience has taught us no such thing: Terrorism is on the rise; there is no record of "swift and sure" retaliation and therefore no evidence that retaliation deters.

Shultz wants us to be willing to use military force and to "understand there is a potential for loss of life of some of our fighting men and . . . of some innocent people." Yet the president recently said that the killing of innocent people would itself be a terrorist act. And Secretary of Defense Weinberger has said publicly that, "Simply unloading a large bombing attack on some group you think may have done it (is) not going to prevent it in the future."

In a comprehensive CBS documentary, serious students of the problem opened up a whole new set of implications, here at home, if retaliation or preemptive strikes against terrorists become a part of policy. FBI Director William Webster said any reprisals that wound up "killing women and children" could provoke "suicide-type" attacks in the United States. Robert Kupperman of the Georgetown Center for Strategic and International Studies said the "infrastructure" for terrorist attacks is already in place inside the United States.

Perhaps the vice president's task force will examine some of the seemingly endless new surprises that might be in store now that the limits of our patience have been reached. If the administration is "drawing the line," the least it could do is tell us what it thinks is on the other side of that line.