"No pictures, no pictures," shouted the black U.S. Marine sergeant to a group of photographers waiting at Ilopango Air Force Base on the outskirts of San Salvador, the capital of El Salvador. The sergeant, a helicopter maintenance mechanic, is understandably camera shy: as one of the 19 Americans sent to El Salvador by the Carter Administration last January, he is a prime target for leftist guerrillas. Anonymity is his best defense.

Those 19 trainers were joined in El Salvador last week by a six-man naval training team that will help repair engines and radar...