Another way to look at it is national defense costs about 2%/year of total assets protected. The world is a dangerous place and 2% is what it costs. More importantly most new technology originally springs from defense spending. America's great wealth comes from using technology to create more value per worker. The whole reason America is the richest country right now is because of our huge investments in defense technology in the 50s and 60s. That 2% is the best use of tax money there is.
I don't know if you can single out defense. I think more knowledge and technology has come out of NASA than the Pentagon.