Foreign interference that may be masked as domestic unrest is another point of concern. If the protesters cannot get near the inaugural events, Department of Homeland Security officials are worried they may turn to cyberinterference, in an effort to black out Mr. Biden’s first words to the nation, and the world. There are similar concerns about infrastructure attacks, perhaps far from the inaugural events, that would convey a sense of chaos, at a moment that Mr. Biden is calling for a restoration of order.
The decision Tuesday night by the Army secretary to arm some of the National Guard troops deploying to Washington 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 and proactive posture to the inauguration than it did last week, when Defense Department officials waited until Trump loyalists overtook the Capitol before deploying additional troops. And even then, those troops that showed up did not carry lethal weapons.
This contrasts sharply to the National Guard response over the spring during the Black Lives Matter protests. Many states called up the National Guard troops, and in Washington, two military helicopters, in a show of force, terrified protesters in Chinatown, producing winds similar to a tropical storm.
The vast majority of military forces in Washington will be National Guard. But a small number of elite commandos from the Joint Special Operations Command, a secretive command based at Fort Bragg, N.C., will be on hand, as they are for every inauguration, to help counter any terrorist attack or other national emergency, especially any involving nuclear, chemical or biological weapons.
Pentagon officials express deep worry about protests that are planned for the inauguration. Some 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 Capitol’s restricted airspace and even remote-controlled drones that could be used to attack the crowd.
A second Pentagon official expressed worry about a repeat of the pipe bombs that were placed in Washington last week. The official said law enforcement agencies were also worried that some protesters had threatened to show up at the homes of lawmakers, or to target their families.
Zolan Kanno-Youngs contributed reporting.