While there are substantial savings to be found in the defense budget, hundreds of billions cannot be cut without impairing our security. Mr. Gates has said that he’s already made the “easy” cuts, yet there are serious questions whether some of them—such as reducing the number of F-22 fighters, Navy cruisers, missile-defense interceptors and strategic delivery systems—leave America ill-prepared for a conventional conflict and erode the strong deterrent necessary to prevent it.

Our country has taken an ax to our national security budget—both the Defense Department and the intelligence community—after every war of the 20th century. And every time we later regretted it. After years of grinding conflict, it can be easy to fall prey to the comfortable fiction that the ugly business of conflict is over and that the U.S. can reduce its military and intelligence capabilities. If we revert to the pennywise policies of the 1990s, we are certain to have to once again scramble to rebuild our defenses in the future. The critical difference between today and past eras, however, is that the proliferation of biological, chemical and even nuclear weapons means that America’s margin for error is considerably more modest.

Defense spending is now 19% of federal outlays and declining. This is the lowest percentage since before World War II. At 4.7% of GDP, the defense budget is dwarfed by the cost of Social Security, Medicare and Medicaid, which exceed 10% of GDP. Even if President Obama tomorrow brought home each and every troop in Iraq and Afghanistan, tore down the Pentagon, shuttered the CIA and the national security agencies of government, and pink-slipped the three million men and women defending the country, it would not solve America’s financial woes.