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The USAF found in Vietnam that CAS was something it couldn't avoid doing, after it kept the US Army from getting the A-4 or a similar aircraft*.
The USAF used the A-1 in the early part of the Vietnam war (along with the F-100), and replaced the F-100 with the A-7D Corsair II at the end of the 1960s. The A-37 was also used (mostly as a replacement for the A-1) from 1968, and was kept in service in the National Guard until it was replaced by the A-10.
The US Army raised a ruckus over the poor quality of CAS provided by the USAF in the early stages of Vietnam, and Congress pressured the USAF to hold a competition for a new CAS aircraft, with an emphasis on anti-armor capability, to combat the Warsaw Pact/USSR superiority in tank numbers in Europe.
Fairchild Republic A-10 Thunderbolt II - Wikipedia, the free encyclopedia
The Fairchild/Republic A-10 beat out the Northrop A-9, and the USAF grudgingly accepted it, and the mission-set that came with it.
Northrop YA-9 - Wikipedia, the free encyclopedia
*
in 1961, two A4D-2 Skyhawks (BuNos 148490 and 148483) were borrowed by the U.S. Army and modified by Douglas for evaluation in competition with the Northrop N-156 (predecessor of the F-5) and an Italian Fiat G-91, for operations from unimproved airfields near front lines. Modifications of the Army Skyhawk included large dual wheels on beefed-up main landing gear mounts; a heavier wing to house the larger landing gear; and installation of an A-3 Skywarrior drag chute. Flown by Douglas test pilot Dru Wood, the modified "Army" Skyhawk won the competition, but the project was canceled when Congress ruled that the US Army couldn't operate fixed-wing combat aircraft, causing Army funds to be diverted to helicopter procurement.
Army Skyhawk | A-4 Skyhawk Association
