The president made good on promises to seek revenge against enemies during his first week back in power, signaling in the process that anyone who crosses him in the future could also suffer.

Dr. Anthony S. Fauci, who was one of the nation’s top health officials for decades, had received death threats during the coronavirus pandemic. He has hired his own security detail.

President Biden used his executive clemency power to protect people targeted by Donald J. Trump, including five members of his family as well as Liz Cheney, Anthony S. Fauci and Mark A. Milley.

The president hasn’t yet decided what he will do, he said, acknowledging that the makeup of Trump’s new administration could influence his decision.