“Today, we’re closer than ever to declaring our independence from a deadly virus,” Biden said to a crowd of mostly unmasked guests gathered on the South Lawn of the White House. “We’ve gained the upper hand against this virus. We can live our lives, our kids can go back to school, our economy is roaring back.”

But in the days after the White House groundskeepers had stowed the white folding chairs and pulled down the paper lanterns, a measure of concern set in among some aides.

“We’re going to pay for it,” one administration official said privately shortly after the event.

Over the next month, the surge of delta cases that overran the country forced Biden and his top aides and Cabinet members to reckon with their overconfidence, which led to a host of decisions — on masks, vaccines and other pivotal issues — that had to be reversed or revised as the crisis spiraled out of control. The administration had been caught flat-footed — and then took weeks to enact a plan in an attempt to catch up.