When the Government was accused of relying on "the politics of fear" last autumn, as it made "safety and security" the main theme of its Queen's Speech, Mr Blair said privately: "If you saw the intelligence reports that came across my desk, you would understand why we do what we do." As far as he was concerned, he would never forgive himself if he ignored warnings that a plane could be shot down at Heathrow or a bomb placed on the underground.
After the Madrid bombings during the Spanish election last March, the Prime Minister feared it was a matter of "when, not if" it happened here. He and his ministers were understandably nervous during this year's general election, which saw the tightest-ever security in a British campaign. When it passed without incident, the word among relieved ministers was that the link between the Spanish bombings and election might, after all, have been a coincidence. That might have been wishful thinking. As the G8 leaders met at Gleneagles, the timing of yesterday's attacks was clearly no coincidence.
Rarely can the fortunes of a prime minister have changed so dramatically in less than 24 hours. The media gave full and deserved credit to Mr Blair's role in winning the 2012 Olympics. He was on a high he had not enjoyed since winning his 1997 landslide. Even Tory MPs shook their heads in admiration. His aides suggested the London Olympics would form part of his "legacy."
After announcing last September he would not fight a fourth election as Labour leader, Mr Blair has, like all prime ministers, been preoccupied with "the legacy question". What was it to be? Transforming public services? Forget it - like painting the Forth Bridge. Resolving Britain's half-in, half-out relationship with Europe? That died when next year's referendum was called off. As the man who forced the European Union to face reality? Perhaps - but it's like turning round an oil tanker. Suddenly, and unexpectedly, the Olympics looked a good bet.
When Blair allies discussed what he might be remembered for, the unspoken word was the one that many people already regard as his legacy: Iraq. After triumph turned to tragedy yesterday, it was a grim reminder that the Iraq cloud still hangs over the Prime Minister, just as it did during the election campaign.
In the short term, the country and its political parties will rightly rally behind Mr Blair. He will show, as he did yesterday, that he is a dependable leader in a crisis, able to speak to, and for, the nation. But he will eventually face some difficult questions, as the anti-war MP George Galloway quickly, and prematurely, reminded him when he said Londoners had "paid the price" for Mr Blair's decision to go to war in Iraq and Afghanistan.
It was not only anti-war campaigners who warned that attacking Iraq could bring reprisals. In 2003, Parliament's Intelligence and Security Committee (ISC) disclosed that five weeks before the war Mr Blair was told by the Joint Intelligence Committee (JIC) that "al-Qa'ida and associated groups continued to represent by far the greatest terrorist threat to Western interests and that threat would be heightened by military action [in Iraq]".
In his evidence to the ISC, Mr Blair admitted "there was obviously a danger that, in attacking Iraq, you ended up provoking the very thing you were trying to avoid". He said he judged that the risk of inaction was worse. "I suppose time will tell whether it's true or it's not true," he said.
All these questions will be weighed up in the months ahead. Yesterdaywill significantly change the terms of the political debate. When the Government puts through new anti-terrorism laws, as it already intended, it might find the backcloth more receptive than it did this spring.
There is also bound to be a debate about whether even more draconian powers are needed to combat a now unmistakable threat. Civil liberties campaigners, who have raised wholly legitimate questions about the Government's proposals, might find it harder to win arguments.
The attacks will also transform the debate about identity cards. Ironically, ministers have recently played down the terrorist threat as a justification for ID cards - originally given as a prime reason. Instead, ministers have talked up the problems from identity fraud and the need to use biometrics to keep Britain up to speed with other countries.
There might now be a new attempt to highlight the terrorism threat in the ID card debate. It may make it more difficult for opponents to campaign against them. Identity cards did not prevent the Madrid bombings- although Charles Clarke, the Home Secretary, argues ID cards did help the police to track down the bombers because the Spanish have to produce their card when buying a mobile phone. He insists that Britain's biometric cards "will be more successful".
It is too early to predict the political fallout from yesterday's events. They may indeed shape Mr Blair's legacy in a way he would not wish. Yet his warnings about the terrorist threat have been justified. As his spokesman said last night: "It was always at the back of our minds. We wished the best, we feared the worst."