‘The war has changed.’ In just days, the U.S. shifts tactics against the virus.

What a difference a week makes. In just a few days, the state and local authorities have imposed mask mandates, companies have put off returning workers to their offices, and the federal government and the military have pressured their employees to get vaccinated.

It started on Tuesday when federal health officials reversed themselves and recommended that even people fully vaccinated against the coronavirus should wear masks again in public indoor spaces in parts of the country where the highly contagious Delta variant of the virus has been surging. They also said everyone in public schools should wear masks, setting off heated debates across the country.

Some state and local governments, schools and businesses scrambled to follow the new guidelines, while others defiantly declared they would not. On Friday, Cornell University directed all faculty, staff, students and visitors to wear masks inside campus buildings and facilities, while Gov. Ron DeSantis of Florida signed an executive order giving parents the final say on whether their children wear masks in school.

By Friday, government documents leaked to the news media had painted a grim picture of the Delta variant as more contagious, more likely to cause severe illness and more able to break through vaccines than other known versions of the virus. One document said officials must recognize that “the war has changed.”

“This virus is in the driver’s seat and we are chasing it,” said Dr. Ali Mokdad, an epidemiologist at the University of Washington and a former C.D.C. scientist. “The only way we stay ahead of it is through vaccination and wearing a mask. If we don’t, we will always be catching up.”

It was a sharp turnaround from the national mood in early July, when President Biden had promised Americans that a more normal life would resume in time for Independence Day parties, describing the holiday as the start of a “summer of freedom.” Instead, in one episode in Provincetown, Mass., an outbreak that began after the town’s Fourth of July festivities has grown to more than 880 cases — almost three-quarters of them among fully vaccinated people.

In recent days, Atlanta and St. Louis have reissued mask mandates that apply to teenagers and adults. An indoor mask mandate was reimposed in Washington on Saturday. Nevada’s new mask mandate went into effect on Friday for counties with substantial or high transmission, which includes Las Vegas. The mandate in Kansas City, Mo., will resume on Monday.

Both New York City and Los Angeles, the nation’s two biggest public school systems, had previously announced they would maintain universal masking policies. This week, the Los Angeles Unified School District took precautions further and announced that weekly coronavirus testing would be required for in-person instruction.

Vaccines are effective against the worst outcomes of infection, even with the variant, reducing the risk of death. But the lagging vaccination effort has fallen behind the ever-evolving virus. Fewer than half of adults are fully vaccinated.

This week, vaccine mandates gained momentum. Mr. Biden announced on Thursday that all civilian federal employees must be vaccinated against the coronavirus or submit to regular testing, social distancing, mask requirements and restrictions on most travel. The Pentagon announced a similar vaccine policy hours later. New York, California, Puerto Rico and others followed suit.

Employers held off for months on making decisions about vaccine mandates, worried about legal and political pushback. But this week ushered in a steady stream of announcements from Walmart, the Walt Disney Company, Google, Facebook, Uber and others introducing new requirements that some employees be vaccinated. And The New York Times indefinitely delayed its return to office, which had been scheduled for September.

“Once you get a little momentum, you get a sort of tidal wave,” said Dr. Ezekiel J. Emanuel, who was a member of Mr. Biden’s Covid-19 Advisory Board during the transition between presidential administrations.

Over the past week, more than 77,200 coronavirus cases, on average, have been reported each day, up 150 percent from two weeks ago, according to a New York Times database. Reported coronavirus-related deaths are up 10 percent, to an average of 301 per day for the past week. Hospitalizations are up to an average of roughly 39,400 per day, a 74 percent increase from two weeks ago.

Experts predicted that cases would peak around mid-August, and that there would be new surges tied to a return to schools and the holiday season this winter.

“We will move past the Delta variant — I suspect — in the next couple of weeks,” said Dr. Céline Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. “But we will most certainly have more waves.”