Niall Ferguson‘s latest Newsweek column takes Republicans to task for buying into an economic argument against battling radical Islamists in the Middle East.

President Obama recently announced a new schedule for scaling down the U.S. military presence in Afghanistan. A total of 10,000 men will come home this year and a further 20,000 by the end of next summer. The surge is over.

This is not a declaration of victory. It is a declaration of bankruptcy. “From a fiscal standpoint, we’re spending too much money on Iraq and Afghanistan,” a senior administration official told The New York Times. “There’s a belief from a fiscal standpoint that this is cannibalizing too much of our spending.”

There was a time when Republicans—and not a few Democrats—would have been dismayed by such a retreat. Yet in their televised debate just a few days before the president’s announcement, the Republican presidential hopefuls vied with one another to out-dove him. …

… The United States certainly needs to get its fiscal house in order. But any serious analysis of the benefits of defense cuts needs to consider the potential costs of walking away from countries like Afghanistan and Iraq. If radical Islamism is a declining force around the world, I hadn’t noticed.

In any case, it’s manifestly untrue to claim that “Bush’s wars” are the principal cause of our current fiscal malaise. The defense budget last year was 4.7 percent of GDP (higher than at any time under Bush), but the cost of Social Security plus Medicare plus Medicaid was 10.3 percent.

Democrats and Republicans alike need to remind themselves of two stark realities. It’s not defense spending that’s bankrupting America; it’s the spiraling cost of entitlements as the baby boomers retire. Meanwhile, the world beyond our borders isn’t getting any safer (just watch Yemen).