I'm gonna' see you and raise you one ... I think it's fair to say the USMC was the first dedicated CAS proponent even prior to WW2 -- they began taking the exercise from experiment into doctrine during the 'Banana Wars' of the '30s ... Haiti, Nicaragua, and the Dominican Republic. During WW2 the concept of CAS was more or less universally embraced by a bunch of players after the Luftwaffe/Blitzkreig proved it's worth on a grand scale', for starters....For the record, the Marine Corps invented CAS during WWII. We're pretty good at it.
But the Marines perfected it.


From the History of USMC Operations in WW2:
"... From the Marine Corps point of view, air support of ground troops by Marine pilots flying Marine aircraft never reached a satisfactory level during the Pacific War. In fact, many World War II Marine aviation commanders considered that their squadrons, groups, and wings were never employed to maximum capability with respect to their tactical functions from a time immediately after Guadalcanal until the Philippines and Okinawa campaigns. In late 1943 and the early months of 1944, tedium and boredom were the order for Marine pilots who, day after day, flew the so-called "Bougainvillea Milk Run," or bombed and re-bombed oft-attacked atolls and islets that were in the backwash of the war. It may have been good experience for young Marine aviators, but it wasn't the type of combat for which they had been trained.
... The situation improved with the assignment of Marine squadrons to the Philippines and Okinawa operations. Attached to MacArthur's forces, the Marines amassed an outstanding record of successful and fruitful close air support missions, and proselytized a number of Army commanders who had not previously been aware of the capabilities of this supporting arm. During Okinawa, close air support was flown for the most part by carrier-based Navy flyers, while Marine pilots of TAF flew combat air patrols and provided the air defense of the island. Although these TAF aviators did the most to blunt the Kamikaze threat and downed a creditable if not entirely confirmed number of enemy planes, they still did not fulfill what has come to be recognized as the primary mission of Marine aviation, close air support of Marine ground troops ... "