May 13, 1962 - United States helicopters carried more than 1,000 South Vietnamese soldiers into the marshlands of the Mekong Delta yesterday in an ambitious attempt to destroy Communist guerrilla nests in the forbidding plain of reeds. But the operation failed to flush any sizable number of rebels. The first reports of 300 Viet Cong dead were drastically reduced this morning. Col. Frank Butner Clay, senior advisor to the Vietnamese Seventh Division, said he doubted that more than 20 guerrillas had been killed. A more modest sortie into the swamp Thursday resulted in the slaying of five Viet Cong and the capture of 10 others, including two 15-year-old messenger boys. Thursday’s operation produced an unusual casualty. A Vietnamese soldier was killed by a water buffalo apparently crazed by the whirring of the helicopters. Three other Government soldiers were felled by the heat.