How the president’s new defense strategy could spark an existential crisis for the Army and the Marines.

It looks like something seriously different is about to happen with the defense budget—and not just the budget, but the way the Pentagon does business and the military fights wars.

President Obama sought to dramatize this fact—or at least to deepen its impression—by going to the Pentagon press room himself (something no previous president had ever done) to lay out the main points of the new “Defense Strategic Guidance,” a document that was hammered out in a half-dozen meetings involving the president, the service chiefs, and the national-security bureaucracy. The precise scope and even nature of these changes are not yet clear. The top Pentagon officials—who followed the president’s appearance with a slightly more expansive press conference—are leaving the details to their budget rollout in a few weeks.

But certain inferences can be drawn from some of their statements. The biggest one is that the Army and Marine Corps will soon be facing an enormous—one might even say, existential—crisis.

Take a close look at these remarks:

Obama: As we look beyond the wars in Iraq and Afghanistan—and the end of long-term nation-building with large military footprints—we’ll be able to ensure our security with smaller conventional ground forces.

Secretary of Defense Leon Panetta: The Army and Marine Corps will no longer need to be sized to support large-scale stability operations.

The president and the secretary aren’t stating merely that they don’t intend to get involved in something like Iraq or Afghanistan in the foreseeable future. They’re saying that when the Army and Marine chiefs calculate how many troops they need for various contingencies, they are not allowed to assume that one of those contingencies might be a long war that involves lots of troops engaged in “stability operations.” (That’s shorthand for operations designed to stabilize a country after a war by helping its leaders impose order, re-establish legitimate government, and provide basic services).

Two big questions come to mind. First, is this the end of counterinsurgency, at least as it relates to much of the doctrine, training, education, and strategy that the Army has pursued during the last five years?

At a follow-on news conference, Ashton Carter, the deputy secretary of defense, waved away such concerns, saying the military would retain the “know-how” and “critical skills” to “regenerate” this capability if it is needed once again. But this is easier said than done. If the president and defense secretary say that “stability operations” are no longer a core mission of the Army and Marine Corps, then the senior officers—many of whom were never thrilled about doing those things to begin with—are likely to whack away at the resources (training bases, educational curricula, and so forth) that created and sustained this “know-how.” (One exception will be Special Operations Forces, which have always performed these sorts of missions and which Obama says he wants to increase.)

The second question: What are the core missions for ground forces now? Panetta said the size of the Army and Marines will be cut substantially, in part because of budget restrictions and in part because of the strategic rethink. News reports say that Panetta will cut the active-duty Army from 570,000 troops to 490,000 troops. But the chiefs had already assumed a post-Iraq reduction to 520,000. Slicing another 30,000 isn’t a big deal. What’s worrying a lot of Army officers is how they’re going to justify even as many as 490,000.