The U.S. could have ignored North Vietnam's assault on the South as a marginal loss on the strategic periphery of the Cold War. Since Iraq is central to the Middle East and one of the three most important Arab countries, al-Qaida could not tolerate our attempt to establish it as a democratic ally in the war on terror. It would have been like the Cold War-era U.S. writing off a Communist takeover of West Germany.
If Vietnam was arguably a winnable war for the U.S. -- once we established a respectable South Vietnamese army backed by our air power -- Iraq was winnable for al-Qaida. In the chaos and civil war it stoked in Iraq in 2006, it came close to collapsing our war effort, and has exacted a stiff price for our intervention there.
The group remains dangerous, and -- if we throw away the gains we've made with a rapid withdrawal -- could mount a comeback in Iraq. Regardless, it still has its redoubt in Western Pakistan. Suffering a Vietnam needn't mean a larger strategic defeat, as we ourselves learned. But the United States had the enormous resources of the world's largest and freest economy, and the essential justness of its cause. Al-Qaida has neither, just the animating hatreds that have been put on such stark, unflattering display during its Vietnam.