For much of the Afghanistan war, the only thing harder for the United States than winning was quitting. The country’s longest war ended in chaos and defeat five years ago Sunday. Hundreds of thousands of Afghans, desperate to escape Taliban insurgents, swarmed the Kabul airport. Thirteen U.S. troops died there in a suicide bombing. But since then, the fears that prevented three presidents from quitting the war have proven to be unfounded. Afghanistan has not become a terror haven or launchpad for attacks in the United States. Today, the U.S. military is mired in a new war in Iran. Once again, the prospects for a military victory look bleak. Even President Trump has hinted that he would like to quit the war, only to reverse course and make a case for staying.

The long war in Afghanistan raises hard questions for the generals and policymakers contemplating the way forward in Iran: Why was quitting in Afghanistan so hard? What does the long struggle to accept defeat there say about the Trump administration’s predicament in Iran? The United States has faced questions about quitting in just about every war it has fought since its emergence as a global power after World War II.

“So much of what we’ve been involved in these last several decades is really policing the empire,” said Andrew R. Hoehn, a senior analyst at RAND Corporation, a think tank that studies security matters. In most instances, U.S. troops have gone to war to impose order on an unruly world, or to defend American values. “We’re not fighting existential conflicts,” Mr. Hoehn said.