Tony Blair has refuted claims that the ongoing crisis in Iraq was the result of the US-led invasion in 2003.
Blair, who was the prime minister during the Iraqi invasion, said pointing fingers at western nations for the crisis is "bizarre".
"We have to liberate ourselves from the notion that 'we' have caused this. We haven't," Blair, who is now an envoy to the Middle East, wrote on his website.
He said the latest conflict in Iraq was caused by the ineffective and sectarian administration of Prime Minister Nouri al-Maliki and the Syrian civil war.
"We have to put aside the differences of the past and act now to save the future. Where the extremists are fighting, they have to be countered hard, with force," he said.
Sunni militants of the Islamic State of Iraq and the Levant (Isis) are waging a deadly war against the Shiite-led government. They have captured several cities and are heading to the capital Baghdad.
Blair's passionate defence through his long essay comes when the US is weighing airstrikes in Iraq. According to reports, the US has ordered its aircraft carrier USS George HW Bush, flanked by two guided missile cruisers, into the Persian Gulf close to Iraq.
Blair said: "On the immediate challenge President Obama is right to put all options on the table in respect of Iraq, including military strikes on the extremists; and right to insist on a change in the way the Iraqi government takes responsibility for the politics of the country."
In conclusion, the former UK prime minister wrote: "Iraq is part of a much bigger picture. By all means argue about the wisdom of earlier decisions. But it is the decisions now that will matter. The choices are all pretty ugly, it is true. But for three years we have watched Syria descend into the abyss and as it is going down, it is slowly but surely wrapping its cords around us, pulling us down with it. We have to put aside the differences of the past and act now to save the future."