After their large-scale introduction to the battlefield in the 1950s and 1960s, helicopters greatly impacted how wars were fought. In 1967, the U.S. military was mired in the Vietnam War, and helicopters were playing a hugely important role. Of the around 12,000 helicopters used by the U.S. armed forces in Vietnam, more than 5,600 were lost, according to figures from the Vietnam Helicopter Pilots Association.