In a strange twist, Tony Blair has become the figure who arouses more passion than any other in UK politics. Blair was the most expedient Prime Minister of modern times. He sought assiduously and persistently to build a big tent of support. He was no Margaret Thatcher or Tony Benn, radicals rooted on the right and left. The twist becomes more bizarre given that Blair applied all his tent-building calculations in relation to Iraq. The war in 2003 was not an aberration. His approach was wholly in character.

Blair speaks now as if he were a free agent when taking the decision to go to war. He was not remotely free. More than a decade ago he did not have to decide whether, personally, he wanted to authorise war in Iraq. He faced an altogether different dilemma: should he support President Bush, who had decided irrationally that after Afghanistan he would turn to Saddam? Blair was in no position to stop the US invasion. That was always going to happen. Bush, Cheney, Rumsfeld and co were determined to act. Blair’s decision was whether to join them or step back from a conflict that would happen anyway.

Blair’s first words outside No 10 after his landslide win in 1997 was that he had won as New Labour and would govern as New Labour. The mindset already established in 1997 led to the decision he took in relation to Iraq. He was resolved to be a different Labour leader, a resolution that formed a superficial but distinct governing philosophy.

Part of the distinctiveness related to defence and the US. Old Labour was perceived to have been soft on defence and anti-US. Blair would be robust on defence and form a strong relationship with any US President. After the 2001 election, before the 9/11 attacks on the US, Blair told his former Culture Secretary, Chris Smith, that one of his main objectives for the second term was to show that a Labour PM could work closely with a Republican President. He would give no space to the new Tory leadership, which had started to claim that it was the party closer to Washington.

In the Spring of 2002 Bush made his “Axis of Evil” speech in which it was clear that Iraq was in his sights. Feted in the US after September 11, Blair faced the biggest dilemma of his career. There was no way he was going to back away from the US now. All his familiar political calculations pointed to him supporting Bush. The Conservatives were gung-ho for war. Most newspapers were supportive of military action. Subsequently Rupert Murdoch’s Sun endorsed Labour in the 2005 election solely because of Blair’s foreign policy. Blair still wanted to be the PM that took the UK into the euro and told friends in the build-up to the war that at least Eurosceptic newspapers would not be able to attack him for being anti-American.

Blair was not the deepest thinker when making policy but he was not daft. In terms of military action he would have been aware of the dangers as well as what he genuinely hoped would be the benefits. But once a decision is taken that leads to war, there can be no public reflection on the dangers. The case had to be made without qualification.

Blair faced obvious difficulties with his own party, so he characteristically attempted a third way. He persuaded President Bush to go to the UN to put his case. Blair also pushed very hard for Bush to focus on Israel/Palestine. As usual he was trying to keep everyone aboard except those well to the left in his own party. His preferred dynamic as a leader was support from some in Labour and the Conservative party, most newspapers and for the left to be wary. If he had opposed the war, the US would still have gone ahead and his favoured big tent of support would have collapsed permanently.

By the time of the massive anti-war march days before the start of the conflict, the course had been set. Blair could only hope that public opinion would change once Saddam had been toppled, but with hostility ongoing he began his long journey from big-tent Prime Minister to unpopular crusader. It was a journey, we should not forget, that was to include another general election victory.

No war leader can admit subsequently that a conflict was a mistake. For a leader who looked to navigate a third way – from the proposed ban on fox hunting that did not abolish fox hunting, to the abolition of hereditary peers that did not quite abolish hereditary peers, Iraq was the ultimate nightmare. He was not strong enough to walk away from Bush but in standing with him he needed to become strong too.

In adapting to unpopularity, in near-exile from the UK, he becomes messianic from a position that began with his usual pragmatism. His inevitable defence of the original war leads him to call for other forms of military intervention now. Trapped by the past, his current views need to be treated with caution.

There are so many lessons, but here are two. The consequences of military intervention must be fully explored in advance and the objectives must be precise. The proposed military action in Syria was dangerously vague and MPs were right to block it.

In terms of political strategy, third ways appear safer than they are. To take a trivial example, Ed Miliband agreed to be photographed in The Sun and then apologised to those who were offended. Either agree to be photographed or disagree. Quite often there are no third ways, and when Blair found his normal route blocked en route to Iraq he had no choice but to become a messianic crusader instead.

London has much to learn from quiet Berlin

I visited Berlin at the weekend and was surprised above all by how quiet the city was. On Friday and Saturday nights the pavements did not heave with hordes of people. Roads were fairly clear of traffic. Cycling was very easy.

When I asked locals to explain the calm, they responded by pointing out that London was unusually crowded while Berlin was closer to the norm. They told me it was only tourists from the UK who were baffled, suggesting that Berlin was more typical of cities with a lot going on but with population levels that did not make the centre the equivalent of a crowded football match.

The contrast was vivid on returning to London on Sunday afternoon. We were told there was no point getting taxis from Heathrow airport because the motorway was jammed for miles. The trains and tubes were packed. London and some other UK cities are great, vibrant places, driving economic recovery. But there are limits to how many more people these overcrowded areas can take. Population levels need some control, including some limited labour market regulation.

The need is so obvious that instead of agonising, for example, over whether there should be any immigration controls at all we should debate what form the regulation should take. The more balanced model of Germany - with its strong regional governments and cities with different national responsibilities - cannot easily be transplanted here. But there are policy implications for the UK in the tranquility of Berlin.