The largest components of US Federal budget outlays are social security, defense and health care, along with the interest on the debt. As this American Heritage article points out we have lost control of the budget process.

"How did the world’s oldest continuously constituted republic lose control of so fundamental a responsibility as its own budget? The answer is, as with most governmental policy disasters in a democracy, one innocuous step at a time. While politicians, economists, and many others pursued their self-interests, the national interest largely got lost in the shuffle.

Over the last sixty years five trends have increasingly affected government fiscal policy. First, a powerful but fundamentally flawed concept in the discipline of economics has completely changed the way both economists and politicians view the national economy and their responsibilities toward it. Second, the responsibilities of government in general and the federal government in particular, as viewed by the public, have greatly increased. Third, a shift in power from the Executive to Congress has balkanized the budget process by sharply limiting the influence of the one politician in Washington whose constituency is national in scope, the President. Fourth, the decay of party discipline and the seniority system within Congress itself has further balkanized the budget process, dividing it among innumerable committees and subcommittees. This has made logrolling (you vote for my program and I’ll vote for yours) the order of the day on Capitol Hill. Finally, the political-action-committee system of financing congressional elections has given greatly increased influence to spending constituencies (often called special interests, especially when they are funding someone else’s campaign) while sharply reducing that of the electorate as a whole, which picks up the tab.

The result is a budget system that has become ever more heavily biased toward spending. As a consequence, the national debt has been spiraling upward, first only in absolute numbers and then, in the last twelve years, as a percentage of the gross national product as well. Today it stands at about 87 percent of the annual GNP, higher than it has ever been in peace-time except in the immediate aftermath of a great war.

To be sure, a country as rich and productive as the United States can well afford to service its present debt. But the current trend is ominous, to put it mildly. Just consider: In the first 204 years of our independence, we took on the burden of a trillion dollars of debt, mostly to fight the wars that made and preserved us a nation. In the last fifteen, however, we have taken on four trillion more for no better reason, when it comes right down to it, than to spare a few hundred people in Washington the political inconvenience of having to say no to one constituent or another."