Though US politics are by no means my expertise I’m willing to give this a shot:

Through the Security act of 18 September 1947 the USAF became an independent Branch of the US armed forces. In practice its predecessor the AAF (Army Air Forces) had become a largely operational independent instrument during WWII already, and the push for an independent air force was even older. (1) In my view this can be explained by the fact that air warfare matured towards the end of WWI and even more so in the interbellum. Military aviation developed its own specific needs , and acquired tasks going beyond the support of ground troops, which was the main focus of the last major conflict involving the use of airpower the US had been in (WWI). Think of things like strategic bombing campaigns, and the Battle of Britain that were fought completely in the Air. Though it is a bit of a chicken and egg story with strategic bombing and independent air forces as often highly placed officers would use the strategic bombing argument, based on Douhet’s doctrine, to gain an independent service branch. One could debate the true impact and effectiveness of conventional strategic bombing as a tool to undermine civilian morale as per Douhetian theory.

William “Billy” Mitchell (b.1879-d.1936) can be seen as the major proponent of an independent US airforce in the Interbellum period, using the argument put forth by Douhet, and setting out to prove that airplanes were capable of sinking ships, so that the air force would supplant the navy as the US’s first line of defense. (2) This rivalry between Navy and Airforce continued throughout WWII and beyond, and arguably still exists today.

Arguably the US was lagging behind a bit with regards to other nations; as the British RAF had been an independent branch since 1918, the German Luftwaffe since 1933 (amongst others). These independent branches showed the enhanced effectiveness of independent air forces.

Following WWII the main task of the airforce, in lieu of IBCM’s, became the nuclear deterrent role. This became the main US line of defence against threats and the only branch capable of fulfilling this role was the air force with its B-29’s. An independent air force was a must in order to carry out this task, and one could argue that dedicated people and the security of funds can only exist through an independent branch, as army or navy officers would be more likely to invest money in things more in line with their own branch like warships. In addition to the aforementioned, WWII and it’s aftermath was costing the US a massive amount of money (Up to 41% of GDP annually) and reforms had to be made in order to cut costs. It was argued that an airforce could deliver destructive power at a much lower cost than any other service branch, and the experiences of WWII seemed to back up this idea.(3)

President Truman believed that a unified department of Defence was a necessity for the Us armed forces to become more effective. The Navy especially agitated against this as this would mean they would have to give up their privileged position and their own Navy Department (which was separate from the War Department!). Truman initially proposed a unification of the armed forces, but only had support from the Army, as a compromise (to gain support from the Navy) the NSC was created: It put the military branches under a single civilian head for the armed forces, as opposed to the old division between navy and war. This civilian head was to become the secretary of Defence, and the 1947 National Security act named James Forrestal of the Navy as the first secretary of Defence. In addition, as mentioned before this act established the USAF as an independent branch and formalized its tasks and objectives putting an end to the argument that had been going on between the army and the navy. (4)

Sources

(1) Conrad C. Crane, American Airpower Stragey in World War II, (Univeristy Press of Kansas, 2016), 28. (2) Ibid. 23-28, Mark Clodfelter, “Pinpointing Devastation: American Air Campaign Planning before Pearl Harbor”, The Journal of Military History Vol.58 No.1 (1994), 75-101. (3) Keith D. McFarland, The 1949 revolt of the Admirals, (1980) http://ssi.armywarcollege.edu/pubs/parameters/Articles/1981/1981%20mcfarland.pdf (4) Anna Kasten Nelson, “President Truman and the Evolution of the National Security Council”, The Journal of American History, Vol72. No.2 (1985), 360-378.