Martin Feldstein, a distinguished economist and a former colleague, made a surprising case for maintaining a large U.S. defense budget, despite a huge federal budget deficit, in the annual Irving Kristol lecture Tuesday night at the American Enterprise Institute.

On one point, he was clearly right: we can afford it. “There is no danger of bankrupting ourselves by so-called ‘imperial overreach’ when we spend less than 5 percent of GDP on defense” (in fact, 5.6 percent of GDP in 2010).

But he failed to make a convincing case that we should spend this much for defense, especially given the dire outlook for federal deficits and the debt. In 2010, U.S. real (inflation-corrected) spending for national security was over twice the annual spending during the Ford and Carter administrations and over 40 percent of total current world defense spending. What conditions, what national objectives, might justify continued U.S. defense spending of this or a higher magnitude?

Feldstein first plays the China card, arguing that “The United States should maintain a military capability such that no future generation of Chinese leaders will consider a military challenge to the United States or consider using military force to intimidate the United States or our allies,” maybe forgetting that a much weaker China successfully challenged us in Korea in the early 1950s. He next makes the case for the importance of a global military presence, arguing that “We have to make it clear by our budgets and by our actions that we are the global force now and will continue to be that in the future.” And finally, “we have to ask ourselves whether we have a moral obligation to defend our allies. …. There are those who say the United States should not be the global policeman. But if not us, who? As the only democratic superpower with the ability to defend and punish, do we not have a moral obligation to be willing to use that power?” All of this assumes without argument or evidence that it is important for the world to have a global policeman, that we can play this role effectively, and that it is a moral obligation for the United States to serve in this role.

The U.S. military had a central role in the most important strategic development since World War II — prevailing in the Cold War against the (former) Soviet Union. But it is critical to recognize that our military has not been very effective as a global policeman or nation builder. The Korean War ended in a draw, leaving a despotic communist government, now with nuclear weapons, in control of North Korea. After 20 years of a U.S. military presence, we abandoned Vietnam to a communist government that now controls most of southeast Asia. The U.S.-sponsored invasion of the Bay of Pigs was defeated, leaving a communist government in control of a large island 90 miles from Florida. U. S. forces have now been in Afghanistan for nearly 10 years without securing it from lightly armed local forces without significant external support. And U.S. forces have now been in Iraq for over eight years without securing it from frequent terrorist attacks.

I wonder what evidence Feldstein or anyone else would offer to support a view that the United States has a comparative advantage as the global policeman. Most of our allies can afford higher defense spending if our support is reduced. The total GDP of the European Union is higher than the U.S. GDP. The GDP of South Korea is many times that of North Korea. There is no obvious calamity that would result if the U.S. contribution to the collective defense with our allies were reduced.

Yes, we can afford a large defense budget, and national security is one of the few federal programs for which there is clear constitutional authority. But like the budgets for most other federal programs, the defense budget is too large. So a substantial reduction of the defense budget should be on the table in any serious effort to avoid a fiscal collapse, a threat that is more serious and more urgent than any that might be effectively countered by trying to maintain the role of a global policeman.