The SEAL trident, also known as “The Bird,” is a hard-earned merit. The mark of a full-fledged SEAL, it can be worn only by those who pass all necessary training and maintain high standards. / Alamy

The most fearsome combat personnel in the U.S. Navy, the U.S. Navy SEALs, evolved from Underwater Demolition Teams first organized on Aug. 15, 1942.

As amphibious landings became a regular part of American offensive operations in Europe and the Pacific during World War II, UDTs preceded landing forces, gathering intelligence and removing natural or created obstacles. After the war, scuba gear became a part of UDT equipment. During the Korean War in the 1950s, parachuting, land combat skills, guerrilla warfare and counterinsurgency were added to the UDT’s repertoire.

President John F. Kennedy’s focus on the importance of unconventional warfare increased the prominence of U.S. Army Special Forces. Around that time, in January 1962, the Navy combined UDT skills in its first Sea, Air and Land teams: SEAL Team One based at Naval Amphibious Base Coronado near San Diego, California, and Seal Team Two at NAB Little Creek, Virginia.

  • SEALs Terry Sullivan, left, and Curtis Ashton capture a Viet Cong in the My Tho area of the Mekong Delta in 1969. / Alamy
  • Members of a SEAL/Underwater Demolition Team are picked up by an H-46 Sea Knight aerial replenishment helicopter as part of a demonstration on April 20, 1970. / U.S. Navy SEAL Museum
  • Trainees crawl across log obstacles during “hell week,” which tests them physically, mentally and emotionally. SEAL training is touted as the toughest in the world. / National Archives
  • One SEAL team member makes his way through deep muck and mud during an operation in May 1970. / U.S. Navy
  • Small craft of the Mobile Riverine Force dock alongside their mothership, the self-propelled barracks ship USS Colleton. / U.S. Navy
  • A SEAL team trains with an inflatable Zodiac boat. / U.S. Navy SEAL Museum
  • Members of Foxtrot Platoon of Sea-Air-Land Team One show off a variety of weapons, including at least three Stoner 63 light machine guns, rejected by other services but popular among the SEALs. / U.S. Navy SEAL Museum
  • SEALs come ashore in a heavily infested enemy area along the Bassac River, 67 miles southeast of Saigon, to conduct Operation Crimson Tide, a one-day sweep against enemy fortifications, bunkers and sampans. / U.S. Navy
  • SEALs board a fire team boat for transportation to their next operation site in October 1968. / U.S. Navy
  • SEALs disembark from an Assault Support Patrol Boat of River Division 91 on the Rach Thom Rach Mo Cay canal system in Kien Hoa province, 50 miles southwest of Saigon, on Jan. 25, 1968. They raided a Viet Cong base, destroying an estimated 40 to 50 bunkers and numerous camp structures, including a propaganda center and two tax collection stations. Additionally, they detained 51 suspected VC. / Naval History and Heritage Command

In May 1983 the last of the UDTs were assimilated as SEAL delivery vehicle teams within Naval Special Warfare Group 3 at Coronado. As with the Special Forces, the SEALs evolved their approach to warfare and sharpened their skills in Vietnam.

The SEALs boast the most demanding physical and mental training in existence.

The Navy declares: “Then comes the hard part — the job of essentially taking on any situation or foe that the world has to offer.” Often tapped to undertake the military’s toughest small-unit direct-action missions, the SEALs have adopted this mantra: “The only easy day was yesterday.” V

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