Theresa May has declared war on terror and changed the rules of the game

It was George W Bush who first used the phrase "war on terror", after the September 11 attacks on the United States in 2001. It was also used by Tony Blair.

In 2005, launching an anti-terror plan a month after the 7/7 London bombings, the then prime minister said: "Let no one be in any doubt that the rules of the game are changing."

Now, in a dramatic late-night statement inside 10 Downing Street after the Manchester bomb attack, Theresa May has both declared war on terror and changed the rules of the game.

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Putting armed troops on the streets and deploying them at major events like this Saturday's FA Cup Final is a declaration of war against terrorists who want to kill and maim sports fans or concert goers.

Terror threat level raised to 'critical'

And raising the threat level from "severe" to "critical" - meaning the threat of another terrorist attack is seen as imminent - means the Prime Minister has spectacularly changed the rules.

Only 24 hours earlier, after an embarrassing General Election U-turn on social care, the Prime Minister was being denounced as "weak and unstable" by opponents who were ridiculing her "strong and stable" boast.

Yet after a frenetic day which saw her chair two Cobra meetings, meet police chiefs and visit injured children in hospital in Manchester, the Prime Minister could not have acted more decisively or swiftly.

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Here, on terrorism, she talked tough and acted tough, with the steel and unflappability she so often deployed as home secretary, unlike in her Tory manifesto wobble on social care.

After six years as home secretary, Mrs May was far more experienced in dealing with national security and the terrorist threat than just about any incoming prime minister in recent history.

But her predecessor, David Cameron, always resisted calls to raise the terror threat to critical, while this Prime Minister - true to her character - has decided to take no chances with the safety of the public.

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Although her visit to Manchester Children's Hospital was "private", according to her official spokesman, she cannot help but have been deeply moved by desperate plight of the injured and their families.

Some of the sights she saw in the children's wards and the stories she will have heard about their terrifying ordeal in the Manchester Arena on Monday night must have been harrowing and distressing.

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Her visit will have persuaded her - if she needed it after the no-doubt chilling briefings from security chiefs on the possibility that Salman Ramadan Abedi was part of a wider group - that drastic action was necessary.

What were her options? In France, President Hollande declared a state of emergency after the terrorist attacks on Paris in 2015 that killed 130 people. The measures give police extended powers of search and arrest.

Mrs May has not gone that far. Indeed, a state of emergency is so rare in the UK that it was last declared by Edward Heath in late 1973 during a miner's strike and three-day week that caused blackouts.

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The terror threat was last raised to critical in 2007 after a blazing car loaded with gas canisters was driven into Glasgow Airport just days after Gordon Brown succeeded Tony Blair as Prime Minister.

Four years earlier, just weeks before the Iraq war in 2003, Mr Blair sent tanks and troops to guard Heathrow Airport after he was briefed that an attack on London was not only probable but imminent.

Sounds familiar? In the next few days, the world's busiest airport could become a military zone once again. But Theresa May won't welcome comparisons with Tony Blair and his "war on terror".