The military can absolutely afford a cut to the budget. However, total spending is around the 600 billion mark. Most estimates of Bernie's plan (while a cost savings overall for public and private) have it around a 3 trillion dollar increase in government spending. Cutting the military budget is more a measure of morality than it is of actual financial consequence. When people say "we'll pay for this by...." and then cite the military, it sorta shows they don't know what they're talking about.
The only way to pay for medicare for all is to tax everyone. People will have to accept transferring their private payments to premiums, co-pays, doctor's bills, etc. into taxes that the government then reallocates (at a cheaper cost) to the medical industry. It's really that simple. But it comes with a difficult sell....give your money to the government to spend it better in the form of taxes.
You don't have to tell me that resisting this is irrational and counterproductive, nevertheless that sentiment is real. And a real impediment in this country to passing that legislation.