The Great Crash by Galbraith is a great source that discusses post WWI American capitalism, as well as the level of raw materials, etc., that precipitated the crash. As for which war most greatly contributed? I'd say it was WWI: American capitalism changed about as much as geopolitical dynamics did after WWI. After WWI, American capitalism simply "piggybacked" after the gains made after WWI. It definitely got a lot more cut throat, but the big changes had already been made.