I think the relationship with the Marine Corps/Navy goes back a long way. The Army was utilizing the Helicopters for a broad range of uses but, the Marine Corps was once under fire from the Government, as to weather we really needed them anymore, or not. If my history is correct, this was most intense after WWII. In an effort to prove their worth, the Marines took it up on themselves to begin using rotary craft as an assault/penetration option. They learned these assault tactics in a very short time, proved their value to the military and, saved themselves. On top of that, they set the standard for helicopter assault tactics and made a hell of a name for themselves, which did not go unnoticed by the top Brass. And, I believe, (correct me if i am wrong), that Johnson was the President at the time. I am not certain on that, though. I don’t want to take anything away from the Army. They have a fantastic Rotary assault program. I have worked on the Blackhawk Program so, I am a huge proponent of the rotary theatre.
I hope that helps. The Book, ‘American Spartans’, by James Warren details out the Marine Corps battle for existence and evolution into the Helo arena.