Interest on the national debt is by far the largest share. Both political parties spend like drunken sailors.
They build used car dealerships and strip clubs within walking distance of bases. Sailors blow thousands in an evening at the club, and then drive home in $75k vehicles purchased at predatory interest rates.
Despite significant, potentially life-changing enlistment and re-enlistment bonuses, housing stipends and more - many (or most) enlistees leave the service in debt or near penniless.