Memories of death and terror haunt him. A boy killed instantly when a grenade blew away his lower body. A woman riddled across the chest with machine-gun fire. Amputations performed on the limbs of the wounded, with no anesthetics. Hiding in the underbrush and taking cover behind a wide tree trunk against a rain of bullets fired from a U.S.-supplied Huey helicopter whirring just above the forest top.

For Philippe Bourgois, a young anthropologist from New York City, that is El Salvador. For 14 days in November he was in the Cabanas province in the northern rim of the country near the Honduran border.

Bourgois, a 1978 Harvard honors graduate, had gone there to do preliminary fieldwork for his doctoral dissertation at Stanford University.

His goal was to interview Salvadoran refugees to learn their political sympathies: why some are with the Farabundi Marti National Liberation Front, some with the government and some with no one.

Before he ever opened his notebook for his first scholarly jotting, Bourgois was caught up in a military offensive waged by a quick-strike brigade of the Salvadoran army. Its leaders had been trained and equipped by American advisers. To avoid what the army would later call a cleanup operation, Bourgois fled for his life with some 700 hungry and poor peasants. Many were women, children and the elderly.

Instead of getting research for his dissertation, Bourgois became a reluctant student of American foreign policy as it affects the people we profess to be most concerned about. Close up, Bourgois saw this policy as a vicious betrayal of the poor. Militarily, he saw its stupidity. The attacks on peasants is assuring the guerrillas of more and more popular support.

At first, Bourgois and the group of 700 thought of fleeing a few hours north to the Honduran refugee camps. But they learned that several hundred Honduran soldiers were stationed along the Lempa River, which divides the two countries. The peasants remembered the massacre of last March when women and children fleeing the Salvadoran army were gunned down by the waiting Honduran army.

The terrified group went south for a few miles. But that route was blocked by the Salvadoran army. So the people hunkered low and tried to keep the children quiet. Bourgois describes the tension of survival: "When a large group of civilians is in flight and hiding as we were, the noise of the crying babies and the moans of the wounded render the entire group vulnerable to detection. The Salvadoran military patrols listen for these sounds. When they hear them, they either radio the position to the helicopters and the artillery or move in themselves to make the kill."

Bourgois and most of his group eventually escaped -- to the north across the Lempa River when the Honduran army dispersed. "The invasion I lived through." Bourgois said when I spoke to him last week, "was a typical one. Dozens of these offensives have taken place throughout rural El Salvador over the past year and a half. It appears that the goal of the military is to annihilate all forms of life -- men, women, children, even farm animals -- where the FMLN is thought to have popular support."

When the Reagan administration speaks loftily of helping the Salvadoran army defend freedom against communism, it in effect sanctions what happens far down the chain of command in the villages in places like Cabanas. Our policy effectively approves the throwing of grenades at children, the machine-gunning of women, the torturing of peasants and the hanging up of mutilated bodies as warnings to others not to help the rebels.

Apparently, though, too many unarmed civilians are escaping, which proves the Pentagon has been right all along: The Salvadoran army needs to be shaped up. Last week, accordingly, 60 of its officers and sergeants were brought to Ft. Bragg, N.C., to be trained ty the Green Berets. Next month, a 1,000-man battalion is coming. Ft. Benning gets 600 Salvadoran soldiers in two weeks.

With better-aiming and better-equipped troops, the ruthless Salvadoran army can be more effective agents of U.S. policy in Latin America: In the name of anti-communism, scorch the earth. And its peasants.