The grin has gone.

The eyes look haunted. The certainty remains. But we have travelled a long way since the days when we believed that with him in charge things could only get better.

I sometimes catch myself feeling sorry for Tony Blair. How crazy is that?

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He is, after all, one of the highest achievers of my generation. He is the most successful leader in the history of the Labour Party; a prime minister three times over.

He brokered peace in Northern Ireland. For me, someone who was born and brought up there, that was a miracle, an affirmation of the power of enlightened politics.

Since leaving office he has become a multi-millionaire with a luxurious country house and another in London. His life as a peace envoy in the Middle East is spent on private jets and he has the ear of influential people. There are those in the world who still rate his opinion on international affairs.

So why should I feel sorry for him?

The answer is because, along the way, he lost his reputation. In my opinion it's the worst sort of amputation. Look at his face and you will see what I mean.

How he has fallen? There are few in even his own country who rate his opinion on international affairs. Can you think of another former prime minister about whom a Mayor of London could be quite so scathing? Writing in a newspaper yesterday, Boris Johnson said, "I think Tony Blair has finally gone mad ... he needs psychiatric help."

Mr Johnson was responding to an essay in which Mr Blair called for us to mount a military response to the jihadist incursion into Iraq.

Mr Blair argued that the carnage Isis is wreaking, the killings and beheadings stem from our previous failure to intervene in Syria. He warned that, if we continue to do nothing, we could have a terror attack in the UK.

Did Mr Blair's former colleagues leap to his defence? Not so far. Even John Prescott, once his Deputy Prime Minister, distanced himself saying Mr Blair was "trying to take the West back to the Crusades".

Nor was there an outcry from the voters who repeatedly elected him into office. With almost half a million Iraqi civilians dead from war-related causes since 2003, there is universal embarrassment at Mr Blair's obdurate insistence that '"We have to liberate ourselves from the notion that we have caused this".

If only that could be justified. But the facts point the other way, as Mr Johnson was quick to remind us.

He outlined with clinical accuracy how, at the time we went to war,I raq had neither an al Qaeda presence nor weapons of mass destruction; nor any involvement in the 9/11 attack on New York. Nonetheless, without a UN mandate, we joined America in "shock and awe".

When the war ended, Mr Johnson - who supported it - visited the empty civil service headquarters in Baghdad. He wrote: 'The days were passing; the city was being looted; no one was showing up for work. We had utterly blitzed the power centres of Iraq with no credible plan for the next stage - and frankly, yes, I do blame Bush and Blair for their unbelievable arrogance in thinking it would work.'

Despite all of this, on Sunday evening I watched Tony Blair make an impassioned case for intervening militarily once again. And once again he based his argument on his convictions.

To my eyes this plea - now dismissed as warmongering and bonkers - differs very little from his original insistence, just over a decade ago, that we take up arms against Saddam.

Then, too, he said we could be attacked - that Iraq could use weapons of mass destruction within 45 minutes of an order being given.

Then, too, his call to arms rested on his belief, his certainty ("the right thing to do") and his rhetoric.

The difference is that, back then, he still had his reputation intact so people believed him. I can remember intelligent, rational, trusting people reading what we now call the dodgy dossier and saying: "The Government must know more than it is telling us." Or "The intelligence services will have secret information they can't publicise for fear of endangering their sources".

It wasn't stupidity that persuaded people to follow their Prime Minister. They followed him because they believed in him. They trusted that he was a leader who espoused Christian family values and brought a moral compass to everything he did. Why wouldn't they? They believed they lived in an ordered democratic society with high standards in public life.

Those who remained sceptical and were unconvinced took to the streets, me among them. Mr Blair railroaded over us.

At the time a leading politician told me: "We go where America goes. It's where our national interest lies." That's realpolitik. I suppose I'd thought Mr Blair was better than that. Surely when it came to war, any responsible human being would step back unless there was no option?

But he had tasted success in Sierra Leone and in Kosovo and had reportedly become convinced of the value of military intervention in pursuit of humanitarian aims.

As we know, he got his war. Saddam was deposed. Iraq descended into hell. And he paid a price. Mr Blair was revealed as the emperor who wore no clothes. His rhetoric was exposed as just hot air or, as some would have it, lies.

Somehow he has survived the Hutton Inquiry and the Iraq War inquiry. We must wait to hear from the Chilcot Inquiry. But along the way he grew ever more tarnished and tinged.

Just before he left office he was challenged about selling peerages. He replied: "I am not going to beg for my character in front of anyone. People can make up their own minds about me."

People have.

His worldly success continues, though he looks like a man who no longer sleeps easy. Until Sunday I'd imagined him tortured by guilt and regret. I thought the innocent dead might haunt his dreams. Now I wonder if he is kept awake by thwarted Messianic fervour.

This latest call to arms suggests he is willing to shed more blood. And the awful truth is that this time he might be right. If al Qaeda gains a power base that stretches across Iraq and Syria, we may all live to regret it.

Barack Obama seems to be contemplating air intervention and Mr Blair clearly thinks Britain should stand shoulder to shoulder again. He might see a second Iraq War as retrospective justification for the first. But even if Britain does become involved, it can never justify that last illegal war.

Mr Blair needs to retreat from the cameras, to climb back on his private jet, go to his influential meetings, sit on his lucrative boards and cash his takings from lecture tours. He can flirt or not flirt with the wives of billionaires. But he needs to stay silent about war. About that he has said too much already.

A leader who has forfeited his reputation should at least have acquired sufficient humility to know when to bow out of the debate.