It's a much tougher undertaking, requiring far more money, knowledge and patience than Americans can muster. We rouse ourselves to ambitious tasks when adversaries challenge us. But as soon as we've taken one down, we lose interest.
American presidents often insist on staying in countries we invade, but they do so in the knowledge that the public's patience and appetite for sacrifice are very limited. So we do our nation-building on the cheap, with an eye on the exit.
That's how we have operated in Afghanistan -- which President Bush shortchanged so he could focus on Iraq, and from which President Obama has promised to begin drawing down our forces in July.
He may not have much choice. A Washington Post-ABC News poll in March found that 64 percent of Americans think the war in Afghanistan has not been worth fighting, and 73 percent favor withdrawal this summer.
And why wouldn't they want out? We have spent 10 years there without showing much progress toward a healthy Afghanistan. The indispensable mission -- defeating those who attacked us -- was largely accomplished by the end of 2001, and it was completed on May 2, 2011.
Chances are slim that we can succeed in transforming a country so alien from our own. When we leave, it may fall back into chaos or even Taliban rule. But that doesn't mean it would pose a threat to us.
The success we had in avenging the 9/11 attacks provides an unforgettable lesson for whoever emerges on top in Afghanistan after we're gone: If you make war on the United States, your destruction will follow. That, we know how to do.