A YEAR AGO, Defense Secretary Robert M. Gates launched an effort to rationalize the Pentagon’s budget in the hope of making military spending sustainable in an era of fiscal crisis. His “greatest fear,” he said last August, “is that in economic tough times people will see the defense budget as the place to solve the nation’s deficit problems.” Last week Mr. Gates learned that his attempt at preventive medicine had fallen short and that his greatest fear might come true. After giving the defense chief 24 hours’ notice, President Obama proposed $400 billion in defense cuts over the next 12 years — in effect doubling the cutbacks that Mr. Gates had identified.

The president said that the reductions would come from across national defense, including the State Department, foreign aid and homeland security, and would be preceded by a new defense spending review. Supporters point out that the proposed cut is considerably less than the $1 trillion over 10 years recommended by the president’s deficit commission; one Democratic defense expert, Gordon Adams, has been arguing that $400 billion could be saved merely by holding the Pentagon budget flat in real terms, instead of including the small annual increase Mr. Obama had previously budgeted.

Still, there is little doubt that Mr. Obama’s proposal would lead to real cuts in “force structure and military capability,” as Mr. Gates put it. His spokesman, Geoff Morrell, said that it will mean identifying “missions the country is willing to have the military forgo.” That should be a sobering consideration at a time when Mr. Obama — a commander in chief obviously loath to embrace military expeditions — has nevertheless embarked on an intervention in Libya, at a cost of more than $600 million so far, after overseeing a costly surge of U.S. forces in Afghanistan.

Defense savings beyond those already achieved by Mr. Gates are certainly possible and even needed — though by and large they lie in areas that Congress has been unwilling to touch. As we pointed out in a recent editorial, military health care now costs as much as the war in Iraq, in part because military families — including working-age retirees — pay one-tenth as much for their health plans as do civilian federal workers. Pay raises for Pentagon civilians have outstripped those in the private sector in the past decade, and the workforce itself has swollen. Though Mr. Gates has succeeded in winning congressional approval for some big cuts in weapons systems, Congress continues to fund planes and equipment — including $8 billion in the new fiscal 2011 budget — that the Pentagon neither needs nor wants.

Even with significant trims in those areas, however, reaching Mr. Obama’s goal would probably require cuts in the size of the Army and Marines beyond the reduction of more than 40,000 troops already proposed by Mr. Gates. Defense analyst Michael O’Hanlon of the Brookings Institution thinks it could require the elimination of more command structures and another round of base closures. What will then happen if the United States is forced into more conflicts like those of the past decade — if it must intervene to prevent Iran’s acquisition of a nuclear weapon or respond to aggression by North Korea, for example?

Mr. Gates, who is expected to leave office this year, said that big defense cuts “would be disastrous in the world environment we see today.” While some reductions in defense are inevitable, that is a warning that the administration and Congress cannot afford to disregard.