In all, $1 of every $5 the federal government spent in 2011 went to defense, and about 20 cents of that $1… — David Wessel

No one really knows how much the U.S. government can borrow before global investors get uneasy and begin to demand higher interest rates. The national debt exceeded 100 percent of GDP during World War II and then came down as the economy sprinted. But history suggests debt of that level is in the danger zone. Think Argentina, circa 2001. Think Greece, circa 2012.

Ultimately, what matters is where Congress and the president end up, not where they start. But defining the starting point and crafting the baseline are important to the politics and public perceptions of the budget—they are used by one side to magnify the size of the spending cuts or tax changes proposed by the other side—and politics and perceptions have a lot to do with what actually happens.

“From the mid-1930s to the 1970s, the government made a set of commitments that led to expectations on the part of the American people about what their government owes them,” says Robert Reischauer, a former director of the Congressional Budget Office. “And they are totally unprepared to go back to a different world.”