In early May 1961, a U.S. military aircraft taxied toward a well-guarded terminal building. The plane slowed to a halt; steps were maneuvered up to its side, and the door was pulled open. The tropical night air was heavy and dank, and the moon shone dimly through high thin clouds. On board the aircraft were ninety-two members of a specially selected team. The men were dressed in indistinguishable dark suits with white shirts and dark ties, and each man carried a new red U.S. diplomatic passport inside his breast pocket. The men held copies of their orders and records in identical brown Manila envelopes, and each man's medical records were stamped "If injured or killed in combat, report as training accident in the Philippines."

In such clandestine fashion, the first fully operational U.S. military unit arrived at Tan Son Nhut Air Base in South Vietnam. The unit was so highly classified even its name was top-secret. It was given a codename, a cover identity to hide the true nature of its mission. The unit's operation was housed in a heavily-guarded compound near Saigon, and within two days of its arrival, Phase I was implemented. Its operatives were intercepting Viet Cong manual Morse communications, analyzing it for the intelligence it contained and passing the information to the U.S. Military Assistance Advisory Group-Vietnam. The Army Security Agency was on duty.