Biden rolls out ‘full-scale, wartime’ coronavirus strategy, including requiring masks on some planes, trains and buses.

President Biden, pledging a “full-scale wartime effort” to combat the coronavirus pandemic, signed a string of executive orders and presidential directives on Thursday aimed at combating the worst public health crisis in a century, including new requirements for masks on interstate planes, trains and buses and for international travelers to quarantine after arriving in the United States.

“History is going to measure whether we are up to the task,” Mr. Biden declared in an appearance in the State Dining Room of the White House, with Vice President Kamala Harris and Dr. Anthony S. Fauci, his chief Covid-19 medical adviser, by his side.

With thousands of Americans dying every day from Covid-19, a national death toll that exceeds 400,000 and a new, more infectious variant of the virus spreading quickly, the pandemic poses the most pressing challenge of Mr. Biden’s early days in office. How he handles it will set the tone for how Americans view his administration going forward, as Mr. Biden himself acknowledged

In a 200-page document released earlier Thursday called “National Strategy for the Covid-19 Response and Pandemic Preparedness,” the new administration outlines the kind of centralized federal response that Democrats have long demanded and President Donald J. Trump refused.

Despite his repeated calls for unity, the new president took a shot at his predecessor, saying, “For the past year we couldn’t rely on the federal government to act with the urgency and focus and coordination that we needed, and we have seen the tragic cost of that failure.”

But the Biden plan is in some respects overly optimistic and in others not ambitious enough, some experts say. It is not clear how he will enforce the quarantine requirement. And his promise to inject 100 million vaccines in his first hundred days is aiming low, since those 100 days should see twice that number of doses available.

Mr. Biden bristled at a reporter’s question when he was asked if the goal should be for a higher number. “When I announced, you all said it’s not possible,” Mr. Biden said. “Come on, give me a break, man.”

Because the currently approved coronavirus vaccines require two doses, but some Americans have already had their first shots, Mr. Biden’s promise should cover 65 to 70 million Americans, said Scott Gottlieb, a former commissioner of the Food and Drug Administration under Mr. Trump.

“I think we can reach that goal and probably reach higher, by focusing on how many people are being vaccinated for the first time each day,” Dr. Gottlieb said. With vaccines by Pfizer and Moderna already granted emergency approval and a third, by Johnson & Johnson, likely to to be authorized soon, he said, “we can definitely reach many more patients.”

Beyond the 100-day mark is where the problem lies. Federal health officials and corporate executives agree that it will be impossible to increase the immediate supply of vaccines before April at the earliest, because of lack of manufacturing capacity.

“The brutal truth is it’s going to take months before we can get the majority of Americans vaccinated,” Mr. Biden said.

It makes political sense for Mr. Biden to lower expectations, and on Capitol Hill, the new president is not getting much of a honeymoon. The No. 2 House Republican, Representative Steve Scalise of Louisiana, said in a statement, “Comments made about vaccine supply and distribution by the White House’s coronavirus czar are old Washington spin.” He added, “The fact is the Biden administration inherited contracts for 300 million doses of vaccines for two approved vaccines and two in the final stage of clinical trials.”

But the Biden team has been quick to point fingers at what they see as the Trump administration’s failures.

“What we’re inheriting is so much worse than we could have imagined,” said Jeff Zients, the new White House Covid-19 response coordinator, adding, “The cooperation or lack of cooperation from the Trump administration has been an impediment. We don’t have the visibility that we would hope to have into supply and allocations.”

In a display of his oft-stated promise to put federal health experts front and center. Mr. Biden was accompanied in the State Dining Room by Dr. Fauci and Mr. Zients. Four other officials participated by video: Xavier Becerra, the nominee for health secretary; Vivek Murthy, the nominee for Surgeon General, Dr. Rochelle Walensky, the new director of the Centers for Disease Control and Prevention; and Dr. Marcella Nunez-Smith, an adviser on racial equity in health.

Efforts to untangle and speed up the distribution of vaccines — perhaps the most pressing challenge for the Biden administration that is also the most promising path forward — will be a desperate race against time, as states across the country have warned that they could run out of doses as early as this weekend.

Though Mr. Biden has indicated his administration would release more doses as they became available and keep fewer in reserve, he said last week that he would not change the recommended timing for second doses: 21 days after the first dose for Pfizer’s vaccine, and 28 days for Moderna’s.

The administration is asking Congress for $1.9 trillion for pandemic relief, and White House officials said they would need much of that money to put their Covid-19 plan into place.