WASHINGTON >> The 365 days between the United States’ panicked retreat from offices and schools and President Joe Biden’s speech tonight, celebrating the prospect of a pandemic’s end, may prove to be one of the most consequential years in American history.

People learned about national vulnerabilities most had never considered, and about depths of resilience they never imagined needing except in wartime. Even the Sept. 11, 2001, attacks, for all their horror and the two decades of war they ushered in, did not change day-to-day life in every city and town in the United States quite the way the coronavirus did.

One president lost his job in large part for mishandling a crisis whose magnitude he first denied. His successor knows his legacy depends on bringing the catastrophe to a swift conclusion.

The halting response demonstrated both the worst of American governance and then, from Operation Warp Speed’s 10-month sprint to vaccines to the frantic pace of inoculations in recent days, the very best. The economic earthquake as cities and towns shuttered so altered politics that Congress did something that would have been unimaginable a year ago this week. Lawmakers spent $5 trillion to dig the nation out of the economic hole created by the virus, and almost as a political aftershock, enacted an expansion of the social safety net larger than any seen since the creation of Medicare nearly 60 years ago.

No country can go through this kind of trauma without being forever changed. There were indelible moments. In the spring came the racial reckoning brought on by the death of George Floyd after a police officer in Minneapolis knelt on his neck for more than eight minutes. On Jan. 6 came the mob attack on the Capitol that led many to wonder whether American democracy was still capable of self-correction.

But Biden’s message on Thursday was expected to center on the thought that the country did finally come together in a common cause — vaccines as the road to normalcy — and from that could spring a glimmer of unity, as a still divided nation seeks solace in millions of tiny jabs in the arm. In his speech, Biden planned to hold out two distinct dates of hope: July 4, when American life might seem a little like it once was, and May 1, when all adults in the United States will be eligible to receive a vaccine.

Jill Lepore, a Harvard historian whose book “These Truths” tracks the changing dynamics of technology and society in America since its discovery, wondered if Americans were unconsciously considering the new year as starting in late March, as it did in Britain and its colonies until the calendar changed in 1752. “Or maybe it begins the day you get your vaccine,” she said. “Or the day enough of us get a vaccine.”

For Biden, the question is when he will be able to pivot from what he has called the “rescue” phase of the pandemic to the “recovery” phase after the pandemic. His aides said that in the speech tonight, the president would make it clear that rescue was still underway.

His goal, his chief of staff, Ron Klain, said in an interview, is “laying up the next steps in this rescue and what, now that we’ve got this bill passed, are we really going to do in the coming months to get back toward a more normal way of life in this country.”

All of Biden’s instincts tell him that declaring a move to recovery too soon carries dangers. It would signal that states could follow the example of Texas, eliminating mask mandates, opening restaurants and bars too quickly, and making themselves vulnerable to a resurgence — what Biden called “Neanderthal thinking.”

His top Cabinet members have emphasized that even eliminating the virus at home is not enough. As his secretary of state, Antony Blinken, said last month, “Unless and until everyone in the world is vaccinated, then no one is really fully safe, because if the virus is out there and continuing to proliferate, it’s also going to be mutating.”

“And if it’s mutating,” he added, “it’s also going to come back and bite people everywhere.”

But the essence of Biden’s message on Thursday evening, aides said, is that for the first time people can begin to imagine a post-COVID world. Americans, they said, can start to think about managing the virus to the point where it does not drive every government decision, every family debate about where to go to dinner, or when to visit grandparents.

All of which raises the question of what will be permanently changed and what, when the history of this national trauma is written, will prove recoverable. And what will the country have learned?

The past provides a mixed guide. There were too few lessons gleaned from the 1918 pandemic, an event that most history books overlooked, and that many Americans first heard about in any detail a century later, when it returned to afflict the nation in a different form. But in 1918, as in 2020, the president’s instinct was to play down its severity, invoking the odd logic that Americans would be dispirited by the truth even as their family and friends succumbed around them.

President Donald Trump has never been a student of history (although his grandfather Frederick Trump died of the flu in 1918), and he told the journalist Bob Woodward that “I wanted to always play it down. I still like playing it down,” because “I don’t want to create a panic.”

No one will know how many thousands of lives that cost as Trump ridiculed mask-wearing and did so little to promote the vaccine in the last days of his administration when it moved from laboratory to market in record time.

Dr. Anthony Fauci, whom Biden deliberately made his top medical adviser, was expected to allude to those unnecessary deaths on Thursday when, on NBC, he is to say “it would have shocked me completely” to know, a year ago this week, that more than half a million Americans would die of the disease. But he planned to note that the country paid a horrific price for its political divisions.

“Even simple common-sense health measures took on a political connotation,” he was expected to say. “It wasn’t a pure public health approach. It was very much influenced by the divisiveness we have in this country.”

When Trump and his wife, Melania Trump, received the vaccine in January, they did not make it public. It was left to Biden and members of his administration to be inoculated on live television as an encouragement to those Americans fearful of the vaccine.

The second big lesson may be that when properly organized, the same government that mobilized for World War II and landed men on the moon can in fact save lives on a mass scale. To the Biden administration, that meant taking the vaccines developed in record time and devising a vital distribution system.

Operation Warp Speed “was very important work, and I don’t mean to minimize it,” Klain said. “But there was no plan for how we were going to get this vaccine into the arms of tens, and ultimately hundreds, of millions of Americans.”

When the history of this strange moment is written, Biden will almost certainly be credited for getting a quarter of the adult population vaccinated with at least one shot, and 10% fully vaccinated, in his first 50 days. After years in which government was denigrated as more of an impediment to national greatness than a vehicle of progress, when conspiracy theories about a pernicious “deep state” still abound, it is a show of government competence.

“What we don’t know is whether that translates into encouraging people into public service, or at least trusting that the government can get something done right,” said Richard Haass, a longtime diplomat and now the president of the Council on Foreign Relations. “After 9/11, we rose to the task of fighting global terrorism. After COVID-19, we rose to a different task.”

“It remains to be seen,” he said, “whether we can now also use the moment to lessen the effects of domestic division.”

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