Behind the uneasy calm lay recognition that something important had shifted. Not just in this city — the latest in Britain to be bludgeoned by terrorism — but across the country, people awoke to a frightening new reality. Another attack could be imminent, Prime Minister Theresa May had warned on Tuesday night, as she put the country on the highest possible alert, raising the threat level from severe to critical.

The move meant that troops would be deployed to buttress police activity, part of a plan known as Operation Temperer. May said members of the armed forces would replace authorities “guarding key sites,” freeing police officers to step up patrols. She suggested that members of the military may also be seen guarding major events, such as concerts and sports matches.

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The extent of the deployment was not immediately clear. May emphasized that all troops would remain under the command of regular police.

The announcement underscored not just the severity of the attack, the deadliest in Britain in more than a decade, but also outstanding security concerns as investigators worked to determine whether Salman Abedi, the man suspected of detonating a homemade explosive outside an Ariana Grande concert at Manchester Arena late Monday, had acted alone or with accomplices. On Wednesday morning, police arrested three men in South Manchester in connection with that investigation.

May's decision also showed the depths to which terrorism carried out by adherents of the Islamic State has roiled Western Europe, altering the landscape and rhythms of public life. Soldiers toting assault rifles could prove a jarring addition to the usual law enforcement presence familiar to Britons. British bobbies have mostly forgone firearms in favor of nightsticks.

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And although May indicated that the threat level was only being raised “for the time being,” she put no clear expiration date on military deployment. It is only the third time that the threat level has reached the critical position in Britain since the system took effect in 2006 — once in response to a plot in 2006 to bomb transatlantic airliners and again the following year after a strike at the Glasgow airport.

“I hope it’s necessary,” said John O’Shea, 41, a manager at a Manchester health company. “Because it does send out a real message — it feels quite threatening.”

Rabnawaz Akbar, a Manchester city councilor, said it was important for the deployment not to stretch on indefinitely. “I think this is, hopefully, a short-term measure that will reassure the public,” he said. “If it continues for a significant period of time, it will be a totally different matter, particularly in a country that doesn’t usually even see their police carrying weapons, never mind the military being out on the streets.”

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So, too, was the uncertainty compounded by the looming general election campaign, which was put mostly on hold this week as its leading contestants agreed to stay off the stump. Some politicians rejected the stay on electoral activities. Mike Gapes, a Labour member of Parliament, pledged to continue campaigning, saying, “We must not allow murderous terrorists to undermine our democratic society.”

Tim Newburn, an expert on law and policing at the London School of Economics, said the election, which takes place June 8, adds to the pressure on the country’s leaders to respond effectively. But he said the attack is unlikely to influence voting, as people have learned to separate terrorism, still rare, from “the ordinary business of daily life.”

The government's response, meanwhile, is unlikely to usher in vast changes in policing practices, he said. “While it’s true that British police officers remain routinely unarmed, it’s not unusual for us to see armed police officers at certain locations, and not only armed officers but armed officers with machine guns and wearing what ostensibly would look like paramilitary style uniforms,” Newburn said. “I suspect they’ll be confined to the places like airports and railway stations. It won’t be as dramatic as it might seem.”

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Initial deployments began Wednesday to prominent government sites, including Buckingham Palace, Parliament and the prime minister's residence on Downing Street.

Newburn said that Britain's heightened security is distinct from the state of emergency in place in France since the 2015 attacks in Paris. That policy, which grants police broad powers of search and detention, has been extended five times, and newly elected French President Emmanuel Macron announced Wednesday he would ask the legislature to authorize a sixth extension, to November.

May's action “is entirely different,” Newburn said. “It’s a temporary and limited reaction in the aftermath of an atrocity, and a situation where the police and the security services are still a little unsure about the likelihood of any further attacks.”

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And while the placement of heavily armed troops in traditionally civilian roles might unnerve some citizens, it is not without precedent in British society, said Ashley Jackson, a military historian at King’s College London.

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The country grew accustomed to the presence of the military during the height of Irish Republican terror in the 1970s and 1980s, he said. Troops were also dispatched to quell labor unrest earlier in the 20th century. And more recently, fearful of retaliatory strikes during the Iraq War, the government posted army personnel at Heathrow Airport, among other locations, Jackson said.