The looming civil war and bloody insurgency in Iraq were caused by the West's failure to intervene in Syria, not the 2003 invasion to topple Saddam Hussein, Tony Blair has said in a renewed call for military action.
Thousands have fled the sweeping advance of fighters from ISIS, Islamists so extreme they were disavowed by Al Qaeda.
The group has have taken control of large areas including second city Mosul, imposed the death penalty by crucifixion for failing to adhere to strict Islamic law and reportedly beheaded a man and published the video of it.
Many have pinned blame for the country's instability on the 2003 invasion and long war in the country but Blair insisted the West's failure to launch another war in Syria was to blame.
Air strikes against the Assad regime had been on the table last summer over its use of chemical weapons but a Commons vote against the idea ended the prospect of action.
Barack Obama, who was elected promising to end America's involvement in Iraq, is considering a range of military options to stop ISIS - said to include air strikes - over the coming days.
Blair said claims Iraq's instability could be linked to the controversial 2003 military intervention there were "bizarre".
"It is a bizarre reading of the cauldron that is the Middle East today, to claim that but for the removal of Saddam, we would not have a crisis," he wrote.
He said that unless the countries had to confront the extremists "hard, with force" and the consequences would be worse still if they did not.
One MP told The Huffington Post UK that denying the link between the Islamist insurgency and the 2003 war, of which Blair was a stern advocate, was "grossly irresponsible".
Blair - now a Middle East peace envoy - said Iraq was "in mortal danger" but blamed the sectarianism of the current Iraqi government, which has alienated the country's Sunni Muslims, and the spread of neighbouring Syria's three-year civil war.
In an eight-page essay on his website, saying al-Qaida had been a "beaten force" in Iraq as recently as three years ago, but the chance of peace was squandered by Baghdad.
"By all means argue about the wisdom of earlier decisions," he wrote.
"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.
"Where the extremists are fighting, they have to be countered hard, with force."
It did not mean another invasion, he said, adding: "There are masses of responses we can make short of that. But they need to know that wherever they're engaged in terror, we will be hitting them."
He said the Iraq assault had been planned and prepared in Syria and was the "predictable and malign effect" of allowing that situation to be exploited by extremists.
"I understand all the reasons following Afghanistan and Iraq why public opinion was so hostile to involvement," Blair wrote.
"Action in Syria did not and need not be as in those military engagements. But every time we put off action, the action we will be forced to take will be ultimately greater."
He wrote: "The moderate and sensible elements of the Syria opposition should be given the support they need; Assad should know he cannot win an outright victory; and the extremist groups, whether in Syria or Iraq, should be targeted, in coordination and with the agreement of the Arab countries.
"However unpalatable this may seem, the alternative is worse."
Green MP Caroline Lucas told HuffPost UK recent events in Iraq could be linked to the "illegal war" in 2003.
She said: "As such, we bear a great deal of responsibility for these most recent events.
“Al Qaeda had very little presence in Iraq before the war. Today, we see militant groups linked to it taking control of major towns and cities.
"It’s an extraordinarily dangerous and worrying situation. To suggest this has no roots in the invasion would be grossly irresponsible and an air-brushing of history."
She added: “The impact of the ‘War on Terror’ dropped off the main news agenda for a very long time – but there has been no let up for the people in Iraq.
The knock-on effects of this war continue to impact daily life there through unemployment, poverty, insecurity, loss and the physical and psychological effects of conflict. Many hundreds of people continue to be killed there every month.”