The elephant in the room is still Medicare and Social Security. Not only are they a much larger share of federal spending, they are growing much faster than the budget as a whole. If we froze spending on these programs today, over the long term the budget would essentially balance itself due to rising tax revenue combined with the slow grower growth of discretionary spending. The CBO models that scenario in their "payable benefits scenario". Remember, Congress has to re-authorize defense spending so the defense budget doesn't grows unless Congress wants it to. Entitlements on the other hand, are on auto-pilot!