I've fantasized about this as a solution before. If you tell the military that in 10 years you're cutting their budget to $250 billion and somehow manage to instill in them that a) you're serious and b) you will not repeal your decision, I have no doubt they would successfully adapt (just like every industry who complains about regulations have ultimately adapted).
I am curious, however, in HOW they would adapt.
I wouldn't want them to drop their research budget (much). Research is almost always good.
For one, you could discharge some of our 1.5 Million active troops [1]. I don't really know why we have that many. That'd measurably cut down on the second largest chunk of the budget (wages) [2].
The highest is 'Operations and Maintenance' and the third highest is 'Procurement'. With a reduction of manpower, they need to procure less. They can retire many assets. I would prefer they NOT sell them to more local police, but thats a different discussion. Procurement could be done MANY times smarter. The F35 is a disaster of design by committee and if each military branch had designed their own plane, I'm sure they'd each have gotten a better one. And, of course, with less money, they have more incentive to push back on the industrial part of the complex for more reasonable prices.
[1] - https://en.wikipedia.org/wiki/United_States_Armed_Forces#Per...
[2] - https://en.wikipedia.org/wiki/File:USA_2010_Military_Budget_... (it's 2010, but the proportions should be similar)