As I understand it:<br />1) NASA wanted a new manned spacecraft after Apollo, to be a cheaper way to fly satellites and probes, and maybe build a space station or Mars ship<br />2) USAF wanted a manned spacecraft so they could be all military-y in space, with manned spy stations, satellites, and maybe research into orbital bombardment (see DynaSoar, RoBo, etc.)<br />3) US Congress said... why pay for two? You and you, make nice and work together. <br /><br />BAM! Space Transportation System.