President-elect Joseph R. Biden Jr. introduced six of the top officials he has selected to serve as part of his national security team, saying that together they would reinstate the United States as a global leader countering terrorism, extremism, the climate crisis and nuclear proliferation.

“America is back”, - Biden, surrounded by the team on a socially distanced stage at The Queen theater in Wilmington, said.

Notably absent from the stage was Biden’s pick to serve as defense secretary. There was no mention of who might fill that post, although Michèle Flournoy, the Pentagon’s top official under President Barack Obama, is seen as a top candidate.

He highlighted a number of “firsts” on the team, including Avril Haines, a former deputy director of the C.I.A, who would be the first woman to serve as director of national intelligence if she is confirmed, and Alejandro Mayorkas, who would be the first Latino and first immigrant to lead the Department of Homeland Security.

Biden noted that Jake Sullivan, his choice to serve as National Security Adviser, would be the youngest person to coordinate the national security team in decades.

And he chose former Secretary of State John Kerry as his climate envoy, a role that does not require Senate confirmation, ensuring that for the first time, a principal on the National Security Council would have a full-time job fighting climate change.

Biden said the selection of Kerry would signal to world leaders that the issue was a priority: “He will have a seat at every table around the world”.

“America is back”, - Linda Thomas-Greenfield, a career diplomat and Biden’s pick for U.S. ambassador to the United Nations, said. “Diplomacy is back”.

Mr. Mayorkas said the Department of Homeland Security had a noble mission in advancing “our proud history as a country of welcome”, a contrast with the Trump administration’s efforts to crack down on all forms of immigration.

Ms. Haines said she had “never shied away from speaking truth to power” and that she accepted the nomination “knowing that you would never want me to do otherwise”.

Antony J. Blinken, Mr. Biden’s choice for secretary of state and one of his closest foreign policy advisers, said he felt a duty to keep America as “the last best hope on Earth”, as it had been for his own family.