The path to beating the coronavirus was clear, but Kelley Vollmar had never felt so helpless.

As the top health official in Missouri’s Jefferson County, Vollmar knew a mandate requiring people to wear masks could help save lives. She pressed the governor’s office to issue a statewide order, and hospital leaders were making a similar push. Even the White House, at a time when President Donald Trump was sometimes mocking people who wore masks, was privately urging the Republican governor to impose a mandate.

Still, Gov. Mike Parson resisted, and in the suburbs of St. Louis, Vollmar found herself under attack. A member of the county health board called her a liar. The sheriff announced that he would not enforce a local mandate. After anti-mask activists posted her address online, Vollmar installed a security system at her home.

“This past year, everything that we’ve done has been questioned,” said Vollmar, whose own mother, 77, died from complications of the coronavirus in December. “It feels like the Lorax from the old Dr. Seuss story: I’m here to save the trees, and nobody is listening.”

For nearly the entire pandemic, political polarization and a rejection of science have stymied the United States’ ability to control the coronavirus. That has been clearest and most damaging at the federal level, where Trump claimed that the virus would “disappear,” clashed with his top scientists and, in a pivotal failure, abdicated responsibility for a pandemic that required a national effort to defeat it, handing key decisions over to states under the assumption that they would take on the fight and get the country back to business.

But governors and local officials who were left in charge of the crisis squandered the little momentum the country had as they sidelined health experts, ignored warnings from their own advisers and, in some cases, stocked their advisory committees with more business representatives than doctors.

Nearly one year since the first known coronavirus case in the United States was announced north of Seattle on Jan. 21, 2020, the full extent of the nation’s failures has come into clear view: The country is hurtling toward 400,000 total deaths, and cases, hospitalizations and deaths have reached record highs, as the nation endures its darkest chapter of the pandemic yet.

The situation has turned dire just as the Trump administration, in its final days, begins to see the fruits of perhaps its biggest coronavirus success, the Operation Warp Speed vaccine program. But already, a lack of federal coordination in distributing doses has emerged as a troubling roadblock.

The incoming president, Joe Biden, has said he will reassert a federal strategy to bring the virus under control, including a call for everyone to wear masks over the next 100 days and a coordinated plan to widen the delivery of vaccines.

“We will manage the hell out of this operation,” Biden said on Friday. “Our administration will lead with science and scientists.”

The strategy signals a shift from the past year, during which the Trump administration largely delegated responsibility for controlling the virus and reopening the economy to 50 governors, fracturing the nation’s response. Interviews with more than 100 health, political and community leaders around the country and a review of emails and other state government records offer a fuller picture of all that went wrong:

>> The severity of the current outbreak can be traced to the rush to reopen last spring. Many governors moved quickly, sometimes acting over the objections of their advisers. The reopenings nationally led to a surge of new infections that grew over time: Never again would the country’s average drop below 20,000 new cases a day.

>> Science was sidelined at every level of government. More than 100 state and local health officials have been fired or have resigned since the beginning of the pandemic. In Florida, leading scientists offered their expertise to the governor’s office but were marginalized, while Gov. Ron DeSantis turned to Dr. Scott W. Atlas, a Trump adviser, and others whose views were embraced in conservative circles but rejected by scores of scientists.

>> While the president publicly downplayed the need for masks, White House officials were privately recommending that certain states with worsening outbreaks require face coverings in public spaces. But records show that at least 26 states ignored recommendations from the White House on masks and other health issues. In South Dakota, Gov. Kristi Noem, boasted to political allies about not requiring masks even as her state was in the midst of an outbreak that became one of the worst in the nation.

Gov. Jared Polis of Colorado said states had faced difficult choices in balancing the virus — often hearing competing voices on how to do it best — and said Trump had left them without the political support they needed as they urged the public to accept masks and social distancing. “The single biggest thing that would have made a difference was the clarity of message from the person at the top,” Polis said in an interview.

The pandemic indeed came with significant challenges, including record unemployment and a dynamic disease that continued to circle the globe. Without a national strategy from the White House, it is unlikely that any state could have fully stopped the pandemic’s spread.

But ​the majority of deaths in the United States have come since the strategies needed to contain it were clear to state leaders, who had a range of options, from mask orders to targeted shutdowns and increased testing. Disparities have emerged between states that took restrictions seriously and those that did not.

America now makes up 4% of the world’s population but accounts for about 20% of global deaths. While Australia, Japan and South Korea showed it was possible to keep deaths low, the United States — armed with wealth, scientific prowess and global power — became the world leader: it now has one of the highest concentrations of deaths, with nearly twice as many reported fatalities as any other country.

Against the odds, some states have managed to keep the virus under control.

Washington state, which recorded 37 of the nation’s first 50 coronavirus deaths, has kept in place a steadily adjusting suite of mitigation measures and now ranks 44th in deaths per capita. If the nation had achieved a rate comparable to Washington’s, about 220,000 fewer people would be dead. Vermont has also been among the states with the fewest deaths, thanks in part to a cautious reopening, significant testing and a mask order.

But a year of political division and uncontrolled coronavirus spread has caught up to most of the country.

In recent days, the virus has been accelerating in nearly every state, and deaths were climbing from Arizona to Connecticut. Even New York, which became a national model for virus restrictions and testing after its spring crisis, is seeing a resurgence.

Winter was always the season in which the virus posed the biggest threat, but in many states, residents have also fallen victim to pandemic fatigue, rendering existing controls less effective.

That has been the case in California, which is now experiencing one of the worst outbreaks in the nation.

The nation’s most populous state was the first to issue a stay-at-home order last spring, and it managed to keep the virus in check for most of the year. But as winter approached, a restlessness set in.

Local journalists exposed how the Democratic leaders Gov. Gavin Newsom and Mayor London Breed of San Francisco — outspoken advocates for virus precautions — had attended birthday parties at the French Laundry restaurant in the Napa Valley, ignoring their own best practices.

Disdain for masks and business closures resonated in more conservative parts of Southern California, and health officials pointed to people who had let their guard down at Thanksgiving as a turning point.

Now federal health officials are warning that a much more contagious variant of the virus could become the dominant source of infection by March, threatening to accelerate the country’s outbreak.

The arrival of vaccines could slow the spread, but the lack of a unified national strategy has resurfaced again as a fundamental flaw. The federal government has pushed the responsibility for administering vaccines to state and local governments, who are strapped for funding and still dealing with daunting virus caseloads. Some states have struggled to deliver the vaccine swiftly, and rules vary widely from state to state.

Biden, who takes office this week, said he would call on the Federal Emergency Management Agency to establish 100 federally supported vaccination centers around the country and would also push for thousands of community and mobile vaccination sites.

But tight supplies will limit how quickly any such plans can be rolled out, and already there are political divisions over whether to trust the vaccine and what social groups should get it first.

Dr. Marissa J. Levine, the director of the Center for Leadership in Public Health Practice at the University of South Florida, said that a failure of leadership — first from the White House, and later from the states — had polarized the entire response to the pandemic and given the virus an extended life. “The toll points to a colossal failure at every level of government,” she said.

The top five worst days for new deaths in the United States have come in January. As the calendar page turned for a new year, the virus was worse than it had ever been.