About 1,100 members of the District of Columbia National Guard — all the members who are available — and 650 Virginia National Guard troops will deploy in Washington on Wednesday night, a National Guard spokesman said.
“The entire D.C. National Guard has been mobilized and is prepared to support law enforcement officers in various locations in the city to protect property, allowing federal and local law enforcement officers to do law enforcement missions,” Capt. Tinashe T. Machona, a spokesman for the D.C. Guard, said in a statement.
The troops are being sent to the D.C. Armory and will be deployed to the Capitol and to other points around Washington, the official said.
In a statement, Jonathan Hoffman, a Pentagon spokesman, said the Guard had been mobilized and the response would be led by the Justice Department. The decision by Army Secretary Ryan McCarthy and Christopher C. Miller, the acting defense secretary, came as a pro-Trump mob breached the Capitol.
Defense and administration officials said it was Vice President Mike Pence, not President Trump, who approved the order to deploy the D.C. National Guard. It was unclear why the president, who incited his supporters to storm the Capitol and who is still the commander in chief, did not give the order. One administration official suggested that the president did not want to be seen calling out the National Guard against his own supporters.
A Defense official said that 150 D.C. National Guard troops were en route to the Capitol at 5:20 p.m. to back up the police in clearing the area. The troops, the official said, would be wearing protective and riot gear but would not be armed, although that could change if the situation deteriorated Wednesday night. The troops would be stationed around the grounds of the Capitol to re-establish a perimeter.
Mr. Miller said on Wednesday afternoon that he had spoken with Mr. Pence, Speaker Nancy Pelosi, Senators Mitch McConnell and Chuck Schumer, and Representative Steny H. Hoyer of Maryland about the situation at the Capitol. He notably did not mention Mr. Trump in his statement.
“We have fully activated the D.C. National Guard to assist federal and local law enforcement as they work to peacefully address the situation,” Mr. Miller said. “We are prepared to provide additional support as necessary and appropriate as requested by local authorities. Our people are sworn to defend the Constitution and our democratic form of government, and they will act accordingly.”
Mr. Hoffman noted that the troops were deploying in “support” of federal law enforcement in the district, reflecting Defense officials’ reluctance to send military troops to the Capitol. Defense officials want the authorities in Washington to use the local police and other law enforcement agencies to confront the mob, with the National Guard troops in support but not in the lead, to avoid the specter of a military battling election protests.
But the tense standoff at the Capitol, and the breach by Trump supporters, led to the decision, officials said.
The F.B.I. also mobilized its officers to reinforce the Capitol Police, a spokesman said.
Bureau agents went to the Capitol grounds on Wednesday to help the police on the scene protect the building and the public. A handful of the F.B.I. agents arrived in camouflage and bearing shields and machine guns late in the afternoon outside the secure location where the senators were being held.
The deployment signaled the growing alarm among federal officials viewing the chaos swirling at the Capitol on Wednesday.
“The F.B.I. has been deployed to assist our U.S. Capitol Police partners, as requested, in protection of federal property and public safety,” the F.B.I. statement said.
The bureau would not say how many of its agents were deployed to the Capitol. But a former government official said that all F.B.I. agents in the region were alerted earlier on Wednesday that they could be called to downtown Washington.