In the Arizona desert, National Guard troops clad in camouflage and toting M-16 rifles conducted round-the-clock patrols at the nation's largest nuclear power plant. On the waterways of Oregon, Coast Guard speedboats scanned riverbanks for any sign of something sinister.
And amid the skyscrapers of New York, where war came home one clear September day, helmeted tactical officers stood guard on Wall Street as a new war unfolded in the sand dunes and streets of Iraq.
The nation battened down under security not seen since the Sept. 11 attacks as U.S. forces attacked on Wednesday night.
Authorities fanned out to power plants, bridges, state capitols and other facilities to shield them against possible retaliatory strikes.
Though most protections had been in place since the terror alert status was raised from "elevated" to "high" Monday night, some states planned to further tighten security now that war has begun.
"The first 96 hours of the war is very important, and it deserves special consideration," said Missouri's homeland security adviser, Tim Daniel.
At the Statehouse in downtown Columbus, Highway Patrol officers searched the bags of people arriving for early-morning appointments Thursday.
"I'm glad they're here -- I feel more secure now that they're checking people," said Barbara Flesher, 49, after chatting with a trooper who checked her bag at a basement entrance. Flesher works in the Ohio Senate clerk's office.
Kansas Gov. Kathleen Sebelius ordered employees at the Capitol complex in Topeka to wear their identification cards and said visitors to the Statehouse and state office buildings would be required to sign in.
"We're taking this seriously," Sebelius said after a conference call Wednesday night in which White House staff and homeland security aides briefed governors on the airstrikes.
The countermeasures were most conspicuous in the two cities terrorists targeted on Sept. 11.
In Washington, the White House was closed to tourists as police used the city's network of 14 closed circuit cameras to monitor activity at landmarks, including the Washington Monument, the Capitol and Union Station.
In New York, police prowled city streets with bomb-sniffing dogs, submachine guns and radiation detectors. Officials worried about suicide bombers and armed takeovers of television stations.
"There is a two-front war here," Mayor Michael Bloomberg said. "One is on the streets of our cities, and one is overseas."
In Iowa, officials arrived at the state's Emergency Operations Center late Wednesday to monitor the first hours of the war and prepare any necessary response. Since Monday, guards have kept a 24-hour watch at the Union Pacific and Burlington Northern Santa Fe rail bridges over the Mississippi River.
Authorities at the North American Aerospace Defense Command and the new Northern Command in Colorado Springs said war could mean more jet fighters patrolling the skies around big cities and ground-based air defenses around population centers.
Airports, schools, truck stops, casinos -- all saw increased security in the days leading up to war.
At Atlanta's Hartsfield International Airport, one of the world's busiest, orange barricades outside parking lots kept cars at least 300 feet from the terminal. In Virginia, Fairfax County public schools canceled all field trips to Washington and New York.
In Berkshire, Ohio, truck driver Dave Worden said he didn't mind the increased inspections at highway weigh stations.
"I'm willing to do what it takes for the country to be protected," Worden said.