War with China or Russia has been the bread and butter of US military planners since then end of WW2. This focus (or obsession?) has meant that the multitude of armed conflicts we have subsequently engaged in, have generally been poorly planned and poorly equipped. The US turns up for low intensity conflicts with high intensity kit (because we can only plan for full scale conflict with China or Russia) and any success depends on the moral fiber and determination of those fighting the battles, not the "skill and intellect" of those planning them back in the Pentagon. The Brits are just as bad at this, its called turning up equipped to fight the last war, not the current one.
The US generally prevails through sheer numbers and determination, no thanks to the piss-poor planning or lack of strategic thinking from the grown-ups. If budgets or socioeconomic factors ever reduce that ability to turn up with more kit (even the wrong kit) than everybody else, success might not be as guaranteed.
PS. The current percentage of those serving in the US Armed Forces is less than 1% of the total population, so it's not really that much of an issue when "only 13% of 18 to 29-year-olds are highly willing to join the military".