For much of the 19th century not including the Civil War, the federal military was kept relatively small as its main focus was to aid in settler moving west and having to only deal with the Natives which didn't requite a large force.
If the Native Americans were actually even less of a threat, perhaps even increased impact from disease than OTL and settlers were mostly unimpeded to progress. Would the US have shifted its focus earlier to expanding across the oceans and trying to keep up with the Europeans militarily?