‘America Is Back,’ Biden Declared in June. The World Is About to Test Him.

Speaking with reporters on Thursday aboard Air Force One en route to Rome, Mr. Sullivan said Mr. Biden would announce plans to modernize stockpiles and “create resilient supply chains.” He said another major focus at the G20 would be securing a communiqué that would outline a plan to protect the globe against future pandemics.

“What the G20 will be focused on is the future,” Mr. Sullivan said. “How do we prevent a future pandemic? How do we ensure that the world is coordinated in a way that it wasn’t sufficiently coordinated when Covid-19 hit last year?”

And Mr. Sullivan said that the president considered the absences of Mr. Putin and Mr. Xi not as an obstacle to coordination but an opportunity to showcase that Western democracies are capable of working together to solve current and future threats. On Saturday, Mr. Biden’s first major meeting of the summit will be focused on economic policy and the pandemic, and then he will meet with the leaders of France, Germany and Britain to discuss ways to get the 2015 nuclear accord with Iran back on track, one of Mr. Biden’s most elusive diplomatic goals since assuming the presidency.

Gita Gopinath, the chief economist at the International Monetary Fund, said that the most urgent economic task for leaders at the G20 summit is slowing the pandemic — in large part, she said, by making good on promises to ship vaccine doses to less wealthy nations that remain critical to supply chains and the global economy but have struggled to gain access to shots.

“To truly end this health crisis and its accompanying economic crisis, we need to get to widespread vaccinations everywhere in the world,” Ms. Gopinath said.

Mr. Biden’s advisers said he was counting on at least one economic policy victory from the conference: the blessing of a global agreement to set minimum levels of corporate taxation, aimed at stopping companies from sheltering income in tax havens like Bermuda.

Mr. Biden’s softer diplomacy skills will be put to the test when he meets with Mr. Macron hours after he arrives in Rome. In September, Mr. Biden’s administration caused a diplomatic row with the French after failing to notify them of its deal to sell nuclear-powered submarines to Australia, an accord that gutted a $66 billion French project to build attack submarines.

The deal, announced as part of a new defense alliance with Britain and Australia, so infuriated the French that Mr. Macron recalled his country’s ambassador to the United States, Philippe Étienne, for several days. American officials, including Antony J. Blinken, the secretary of state, have streamed through France trying to repair the damage. Vice President Kamala Harris is due in Paris in November.

Célia Belin, a visiting fellow in the Center on the United States and Europe at the Brookings Institution, said in an interview that the Biden administration seemed to forget “the tasteful art of diplomacy.”

Since the blowup, Mr. Macron has spoken openly about the need for European nations to practice so-called strategic autonomy, a policy that would gradually mean countries like France would rely less on the United States for military assistance. As the summit gets underway, Europeans will be watching with interest about whether Mr. Biden supports such an initiative, Ms. Belin said.

Mr. Biden is also expected to address at least one more rocky diplomatic relationship while abroad. In recent days, President Recep Tayyip Erdogan of Turkey has threatened to recall its ambassador to the United States after several embassies, including America’s, called for the release of an imprisoned activist.

Mr. Sullivan, the national security adviser, said he anticipated that Mr. Biden would meet Mr. Erdogan in Scotland.