When and how did America actually started modernise her armed forces esp. with reference to WW2?

Answer 1

With the Election of FDR in 1932. Roosevelt used the recovery to modernize the US Navy buy starting the new classes if Carriers,Battleships, Cruisers and Destroyers. While The Army was used to run the CCC in 32 and by 1935 Marshall was appointed Chief of Staff he began to modernize the Army and the Air Force. Marshall Used planes that he had set down in the 1920's to change the way that the US Army was laid out. He set up the triangulated division and bought new weapons and planned new camps. Read Geoffrey Perett's There's a War to be Won and Winged Victory and Edward Beaches Peoples Navy for more information.

Answer 2

If a particular event has to be found, I would point to the building of the aircraft carriers Lexington and Saratoga in 1927. Prior to this a number of events were pushing military aviation, including the bombing tests on ships by Billy Mitchell in 1921 and the various air races held in the late 20's and early 30's which produced prototypes of fighters used in WW2. In the ground forces, the war games in 1941 Louisiana Maneuvers in which Eisenhower participated and produced planning changes mark the culmination of changes, including replacing horses and mules with mechanical power, but these were very late. In fact, much of the modernization came very late, including the emphasis on carriers forced by sinking so many battleships at Pearl Harbor and the emphasis on tanks following the British battles by Montgomery in North Africa.