None of that spending is subject to that much debate; all the remaining "debate" is over the remaining 6%.
I don't think defense is really as discretionary as it seems. A lot of it is effectively bribing and menacing trading partners to keep trading with the US on favorable terms through cash transfers, provision of military equipment, training, and mutual defense pacts among other diplomatic agreements.
Japan didn't just decide on its own free will to become a pacifist country dedicated to exporting cheap, high-quality manufactured goods to the United States. General MacArthur did that.
That's just a byproduct of propping up the military industrial complex; have to have conflicts to justify giving the primes sole-source cost+ contracts so they can meet their quarterly EPS targets.