United States Navy SEALs are members of a special operations unit in the armed forces. They are rigorously trained for tactical missions that are often quite difficult and dangerous. The acronym SEAL stands for sea, air and land. Of all the special operations forces in the U.S. military, Navy SEALs receive the lengthiest and possibly the toughest training. For more than 50 years, SEALs have carried out special military operations, from combat in Vietnam to the killing of Osama bin Laden.

Origin of the U.S. Navy SEALs

Established in early 1962 at the request of President John F. Kennedy, the first Navy SEALs were recruits from underwater demolition teams. Their mission was to combat guerilla warfare and conduct clandestine operations in hostile territory, specifically marine and riverine environments. Their punishing training and infamous Hell Week were adapted from the rigorous exercise drills of World War II Navy divers. In late 1962, SEALs went to Vietnam to advise South Vietnamese forces. South Vietnam's non-communists were fighting both the communist military of North Vietnam and a guerilla organization in South Vietnam called the Viet Cong. Within a few years, the SEALs were not merely advising but engaged in direct combat. Navy SEALs led combat operations in Vietnam from 1965 through 1972.

The Phoenix Program

A secret CIA operation, the Phoenix program, was implemented in Vietnam during the mid-1960s. It was a highly classified strategy aimed at destroying the Viet Cong's political and paramilitary infrastructure. Under the CIA's Phoenix program, a group of soldiers called Provincial Reconnaissance Units, or PRUs, captured and killed Viet Cong guerillas. Navy SEALs worked closely with PRUs, which consisted of allied military and paramilitary fighters, including Marines and Army Special Forces soldiers. While deployed in Vietnam, the SEALs shifted their tactical focus to the terrain of Southeast Asia and perfected their jungle warfare skills. With the help of Navy SEALs, the PRUs became the deadliest and most effective fighters in South Vietnam. The Phoenix program continued until 1972.

The Men with Green Faces

By 1968, the Navy SEALs were operating in 12-man platoons, with each platoon consisting of two six-man squadrons. Most SEAL missions in Vietnam were stealthy and extremely dangerous; therefore a small team could get in and out quickly. For example, SEALs carried out night ambushes, taking the enemy by surprise. They also attacked enemy targets by boat and helicopter. Using both Army and Navy helicopters, SEALs developed hit-and-run assault maneuvers. Viet Cong guerillas dubbed the SEALs "the men with green faces" because of the camouflage face paint they wore during raids. The Viet Cong were fearful of SEALs and often issued rewards for their capture.

The Tet Offensive

The Tet Offensive was a series of attacks launched by the Viet Cong in 1968. Despite intelligence warnings, the South Vietnamese and U.S. forces were stunned by the onslaught of attacks. Analyzing the outcome, the U.S. Navy concluded that SEALs should have been better utilized for intelligence purposes. SEALs did gather invaluable intelligence on their missions, but they were often misused as undercover infantry soldiers. Instead of using SEALs for espionage, the Navy sent them to destroy specific enemy targets. With the Tet Offensive, the Viet Cong tried to alter the course of the war -- and they succeeded. By 1970, the U.S. resolved to withdraw troops from Vietnam. But while conventional armed forces were leaving, SEAL operations did not end until 1972. The last Navy SEAL left Vietnam in March 1973.