IT WAS a quiet evening in the sleepy little town of Bien Hoa 20 miles north of Saigon, base camp fir the South Vietnamese crack 7th Infantry Division and its eight-man U.S. Military Assistance Advisory Group. The presence of the Americans symbolized one of the main reasons why South Viet Nam, five years ago a new nation with little life expectancy, is still independent and free and getting stronger all the time—to the growing chagrin of Communists in neighboring North Viet Nam. Since the beginning of 1959, Communist infiltrators have stepped up their campaign of terrorism, assassinating an average of one South Vietnamese a day, frequently hammering lonely victims to death and then hanging their battered bodies in trees under a red flag. But not since 1957 had the Communists dared attack any Americans.