Krugman is critical of our current defense budget but doesn't consider it "the heart of the budget issue":
The current Obama budget calls for defense spending of 3.4% of GDP by 2016; you can make the case that the number should be closer to 2%. But that’s not enough to avoid hard choices about health care and revenue. If you can’t see how it’s possible both to believe that we waste a lot of money on the military, and to believe that ending that waste would make only a modest contribution to our fiscal problem, I can’t help you.
Yes, and means testing Medicare won't fix the budget. Nor will taxing the bejesus out of the rich. Which is why all of these reforms, and many others, must be done in concert.