Two thirds of the budget is non-discretionary funding..that is programs that have specific funding sources`(taxes) rather than general revenue, like Social Security and Medicare, as well as interest on the national debt and Congress and the WH cant (or are not supposed to) mess with it.
Most of the 65% Human Resources in ace's chart is non-discretionary spending (it should be broken out between discretionary and non-discretionary)... along with the 11% net interest (interest on the debt).
That leaves about 1/3 of the total annual budget as discretionary, meaning Congress and the WH get to fight about how it is divided up.
Defense is the largest part of discretionary funding by far...generally about half of all discretionary funding... and the other half for all other other domestic programs, etc.
Non-discretionary (mandatory) spending is out of control and needs a serious fix; but part of the blip in red in the last 6 years is due to much higher interest payments as a result of the national debt doubling in those six years.
The blue shows how discretionary spending was reasonably stable, with the blip coming in fy 2002 - fy 2007`when Bush and the "fiscal" conservative Congress starting spending more.