The best way to reduce defense spending is to increase taxes to pay for it and to give the military an incentive to end wars rather than provide a blank check for endless.
The military does what it's ordered to do by the civilian government and under what rules of engagement it has to operate.
Unleashed from those constrains, our military could've ended Iraq and Afghanistan in months, not years, but we try to fight "nice" wars now.
Of course what would happen is the government would collect the tax, then siphon off the money for other things. Like they did for Social Security, and DEFENSE!