Faulty guidance from health authorities, a cultural aversion to masks and a deeply polarized politics have all contributed. So has a president who resisted role modeling the benefits of face coverings, and who belittled those who did.

The result, experts say, is a country that squandered one of its best opportunities to beat back the coronavirus pandemic this spring and summer. In the process, the United States fell far behind other nations that skipped the fuss over masks, costing lives and jeopardizing the recovery heading into the fall.

“Some countries took out their masks as soon as this happened,” said Monica Gandhi, an infectious-disease specialist from the University of California at San Francisco, “and their rates of death are very low.”

In a coronavirus response that has been full of missteps and unforced errors, delayed acceptance of universal masking, Gandhi said, may be the single biggest mistake the United States has made.