The way the bill treats defense is fairly confusing. The White House told Foreign Policy’s Josh Rogin that in the first round, there are roughly $420 billion in cuts over 10 years to “security” spending (which includes the Pentagon, Homeland Security, State Department, Veterans Affairs and USAID), compared with the baseline. Of that, $350 billion is supposed to come out of the Pentagon’s pockets. But as Rogin points out, the White House was reticent on the fact that that $350 billion number wasn’t a sure thing. It’s up to Congress, not written into the bill.

Take the near term. In fiscal 2011, the total security budget was $689 billion, of which $529 billion went to the Pentagon. Next year, under the debt deal, security spending gets capped at $684 billion. But there’s no guarantee that the Pentagon will absorb that $5 billion cut. In fact, if — as many hawks in Congress would prefer — the Pentagon’s budget grows next year, then agencies such as the State Department and Homeland Security will have to absorb even more in cuts.

“We just don’t know how that will play out yet,” says Gordon Adams, a senior White House budget official for national security in the Clinton administration.

Now, in the medium term, assume that the Pentagon does have to take a $350 billion hit over 10 years. That’s not significantly different from President Obama’s budget proposal this year, which called for $400 billion worth of defense cuts over 12 years. At a Thursday hearing at the House Armed Services Committee, the vice chiefs of the Army, Navy and Air Force argued that that was about the outer limit of what they could handle.

An April analysis from the Stimson Center, on the other hand, argued that a $400 billion defense cut from the baseline over 10 years would essentially mean letting the Pentagon’s budget grow with inflation starting in 2011 — and that’s not much of a cut at all, given the massive run-up in defense spending over the past decade. To put that in perspective, says Center for American Progress defense analyst Larry Korb, “We’re already spending more in adjusted dollars than we have at any point during World War II — more than when we were in Vietnam and had 500,000 people on the ground.”

Of course, there’s still the potential for deeper cuts. Under the debt deal, if the new “super committee” fails to pass its deficit plan, then $1.2 trillion of cuts come down, of which roughly half fall on security — though, again, the details would be left to Congress. That would bring us into the world contemplated by Tom Coburn, in which defense faces nearly $1 trillion in cuts over the next decade. That would certainly garner hostility from defense hawks. But it would also put us squarely in the historical norm.

Adams provides some context. The United States has had three military “build-downs” since World War II — after Korea, after Vietnam and after the Cold War. “With Iraq and Afghanistan winding down, we’re in another build-down,” he says. He notes that between 1985 and 1996, with the end of the Cold War, military spending declined 36 percent. By comparison, a $1 trillion cut in the next decade would represent a 15 percent decline. “Compared with a $350 billion cut, that’s harder labor, definitely,” Adams says. “Is it impossible? No.”

A related question is whether future Congresses would abide by the cuts. The debt-ceiling bill, for one, makes clear that the caps don’t affect emergency supplemental spending. That’s supposed to be used for one-time, temporary expenditures such as wars; in the 1990s, for instance, when defense caps were in place, Congress still found extra money for the wars in the Balkans. But supplementals give those in favor of defense spending extra flexibility — there was some debate in the Iraq and Afghanistan supplementals about whether money was going toward equipment that the Pentagon would’ve bought anyway. “Do they fudge it sometimes? Yeah! This is Washington,” Adams notes.

Plus, of course, defense caps can always be lifted or modified by future congresses. This happened after the 1985 Gramm-Rudman-Hollings deficit deal tried to tighten the Pentagon’s belt. And in 1998, as soon as budget surpluses started appearing, defense officials quickly put pressure on Congress to boost defense spending. As Jonathan Allen quips in Politico today, “it's safe to say defense caps and cuts aren't as hard and fast as domestic cuts and caps.” Or, as Korb puts it: “It’s one thing to put caps on Social Security. Congress can enforce that. But to say we’ll be able to dictate what defense spending will look like in 2018 — that’s a lot harder to do. Who knows what the world will look like then?”