People
The U.S. military expends great amounts of time and money on training recruits, with good results. Each service branch maintains it's own training schools in addition to interservice schools. Any reasonably bright, literate teenager can be transformed into any kind of clerk, technician, a medic, or a human version of 'The Terminator'.
Officers are even more highly trained. Starting with college graduates the services turn out pilots, commanders, intelligence officers, logistics officers, and infantry officers.
Retention of all types of personnel is reasonably good, adding to an experience pool that the next round of recruits can draw on.