The argument for leaving 10,000 U.S. troops in Afghanistan after 2014 is more or less reasonable on its face. The Kabul government is fragile; our gains might be reversed; the Afghan military is not ready to stand on its own. Here's the unreasonable, unavoidable part: If we don't leave then, we probably never will.

The lesson of the past several decades is that once Americans establish ourselves to assure security, we stay as long as it takes and then stay some more. World War II has been over for 67 years, but we still have 37,000 troops in Japan and 53,000 in Germany.

At one time, these forces could be justified as a counterweight to the Soviet Union, but the Cold War is ancient history. The Korean War ended in 1953, yet 28,500 American troops remain in South Korea.

Going over the fiscal cliff may not be good for the economy, but it might have one valuable result: forcing Americans to reassess our enormous defense budget.

Taking $492 billion away from the Pentagon over the next decade wouldn't be hard to do if we forced other nations to take more responsibility for their own defense—and used the opportunity to reduce our overall troop strength. What's hard, and expensive, is our vast array of overseas commitments.

Why do we maintain these deployments? Partly out of inertia, partly out of a feeling they can't do any harm and partly from an incessant fear that anything that happens anywhere poses a potential danger to our security.

This last factor is hard to overstate. Earlier this year, Dartmouth College political scientist Benjamin Valentino constructed a poll that was carried out by YouGov. When respondents were asked if they think the United States "faces greater threats to its security today than it did during the Cold War," 63 percent said it does, with only 14 percent disagreeing.

"It's astonishing to me," Valentino told me in an interview in his campus office last month. Not only are we no longer under the constant threat of nuclear annihilation, he notes, but we have few actual adversaries. Many Americans are aware that we spend more on the military than the next 17 countries combined. What they may not realize, says Valentino, is that "of the next 10 biggest military spenders, all but two (Russia and China) are allies." You have to go to No. 25, Iran, to find a real enemy.

In world history, he says, "there is no precedent for the strongest power to have allies among so many other military powers. Russia and China are only quasi-adversaries." Iran and North Korea are military pipsqueaks, with or without nuclear weapons.

Al-Qaida is a terrorist threat, but it never had any hope of defeating us—only of terrorizing us. And it hasn't been able to carry out an attack on U.S. soil since 9/11.

Our enviable strategic position gives us plenty of room to reduce defense outlays without compromising our safety or inviting attacks on our allies. It's hard to see any remaining military threat to Germany or the other countries of Western Europe that our forces ostensibly protect. Nor do we need troops there for one of NATO's original purposes: to keep the Germans under firm control.

Japan and South Korea may face genuine threats (China and North Korea), but they have ample resources to manage them. Japan has the world's third-biggest economy. South Korea's economy, which ranks 15th, is 80 times bigger than North Korea's.

But our allies punch below their weight. The U.S. spends 4.7 percent of its gross domestic product on defense. Japan spends just 1 percent, and South Korea 2.8 percent. In Germany, the figure is 1.3 percent. They have no reason to spend more as long as they can free-ride.

Would our permanent pullback from Europe and Asia change the strategic environment? Certainly. But after decades of American protection, our friends can form their own alliances to confront any adversary, present or future.

Worries about China and uncertainty about U.S. intentions have already moved Japan in that direction. "We want to build our own coalition of the willing in Asia to prevent China from just running over us," Yoshihide Soeya, director of the Institute of East Asian Studies at Keio University, told The New York Times.

We could foster more such efforts by our allies to work together to defend themselves. Or we could go broke doing it for them.