Allowing the enduring controversy over the decision to invade Iraq in 2003 to hold back military intervention in Syria could help produce a "nightmare scenario" for the West in the Middle East, Tony Blair warned.
The ex-prime minister who took the decision for British troops to join the US-led action, who is now the Middle East peace envoy for the US, Russia, the EU and the United Nations, said it was vital to "take sides" against the Assad regime and in other regional disputes.
Drawing on his own experiences, Mr Blair wrote in The Times: "I know as one of the architects of policy after 9/11 the controversy, anguish and cost of the decisions taken.
"I understand why, now, the pendulum has swung so heavily the other way. But it is not necessary to revert to that policy to make a difference. And the forces that made those interventions in Iraq and Afghanistan so difficult are of course the very forces at the heart of the storm today.
"They have to be defeated. We should defeat them, however long it takes because otherwise they will not disappear. They will grow stronger until, at a later time, there will be another crossroads and this time there will be no choice."
He went on: "After the long and painful campaigns in Iraq and Afghanistan, I understand every impulse to stay clear of the turmoil, to watch but not to intervene, to ratchet up language but not to engage in the hard, even harsh business of changing reality on the ground.
"But we have collectively to understand the consequences of wringing our hands instead of putting them to work.
"People wince at the thought of intervention. But contemplate the future consequence of inaction and shudder: Syria mired in carnage between the brutality of Assad and various affiliates of al Qaida, a breeding ground of extremism infinitely more dangerous than Afghanistan in the 1990s; Egypt in chaos, with the West, however unfairly, looking as if it is giving succour to those who would turn it into a Sunni version of Iran.
"Iran still - despite its new president - a theocratic dictatorship, with a nuclear bomb. Our allies dismayed. Our enemies emboldened. Ourselves in confusion. This is a nightmare scenario but it is not far-fetched."
On Syria, he wrote: "I hear people talking as if there was nothing we could do: the Syrian defence systems are too powerful, the issues too complex, and in any event, why take sides since they're all as bad as each other?
"But others are taking sides. They're not terrified of the prospect of intervention. They're intervening. To support an assault on civilians not seen since the dark days of Saddam.
"It is time we took a side: the side of the people who want what we want; who see our societies for all their faults as something to admire; who know that they should not be faced with a choice between tyranny and theocracy."
Mr Blair said it was a "grave strategic error" to ostracise the interim government in Egypt as the policy was based on treating the Muslim Brotherhood as a political party not a movement which sought to "subvert" the state in "pursuit of values that contradict everything we stand for".
"You can rightly criticise actions or overreactions of the new military government but it is quite hard to criticise the intervention that brought it into being," he suggested.