Mr. McCarthy’s decision came after a meeting with Speaker Nancy Pelosi, Democrat of California. Ms. Pelosi, according to congressional staff members, demanded that the Pentagon take a more muscular posture after a mob, egged on by President Trump last week, breached the Capitol.

Pentagon officials say they are deeply worried about protests that are planned for the inauguration of President-elect Joseph R. Biden Jr. next week. About 16 groups — some of them saying they will be armed and most of them made up of hard-line supporters of Mr. Trump — have registered to stage protests in Washington, officials said.

One Defense Department official said law enforcement agencies are planning for a range of outcomes, including a worse-case scenario in which people with firearms try to attack dignitaries, “suicide-type aircraft” try to fly into the capital’s restricted airspace and even remote-controlled drones that could be used to attack the crowd.

While those scenarios are among the more extreme, the officials said they were particularly worried about the possibility of multiple, violent confrontations, including with firearms, simultaneously flaring up around the inaugural dignitaries.