UDT activated during WWII. During Vietnam the group took on missions from air and land and part of UST became SEAL - SEa Air Land. The teams were very effective in jungle operations because of the previous training in field expedients such as explosives, booby traps, and jungle warfare. USN began the SEAL training in 1966 with jungle survival, intensified h2h, and other Vietnam type stuff. Certain offshoot groups were formed that were more specialized and more highly trained - sort of cross-trained - that were outside the SEAL groups but went through some of their training and added much other training. Those types were discontinued shortly after they started because of the cost of training (cost more to train some of them than it cost to train a pilot). In the 1980s, one of the earlier SEALs was commissioned to develop a training course that began the types of training now. BUDS was used, and more diversity with weapons became standard. During Vietnam the UDT and SEAL teams became more or less separate entities.