Max Boot is a senior fellow at the Council on Foreign Relations. He is completing a history of guerrilla warfare and terrorism.

The signature line of President Obama's June 22 Afghanistan address was "America, it is time to focus on nation-building here at home." This no doubt resonates among an electorate sick of foreign wars and eager to focus on domestic problems, but it is a wrongheaded statement.

Whenever America has eschewed commitments abroad and turned inward, the results have been disastrous. The most isolationist decade in the country's history — the 1930s — was followed by World War II. The "Come Home, America" isolationism of the 1970s was followed by the fall of South Vietnam, the genocide in Cambodia, the Iranian hostage crisis and the Soviet invasion of Afghanistan. In the 1990s, the post-Cold War desire to spend the "peace dividend" led the U.S. to turn a blind eye to the rising threat from Al Qaeda.