Blair, now a Middle East peace envoy, rejected what he called "bizarre" arguments that Iraq would be more stable and peaceful today without the US-UK invasion in 2003, and said the international community must overcome public reluctance to confront extremists beseiging the country.
Blair's comments come days after a retired Major General Julian Thompson, a commander during the Falklands war, described the original invasion as "a strategic error of gargantuan proportions".
Blair rebuffed suggestions that the Iraq war, which demolished Saddam Hussein's secular regime, was at the root of the current chaos.
In an eight-page essay published on his website today, the former PM pinned the blame on the sectarianism of the Maliki government and the spread of Syria's brutal three-year civil war. He said: "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."
This could include air strikes, he added, rather than a new ground invasion.
Foreign Secretary William Hague has said Britain could offer counter-terrorism expertise, while former Defence Secretary Liam Fox said the idea of sending troops back to Iraq should not be ruled out.
Meanwhile, the UK's Department for International Development announced £3 million in emergency aid, including clean water and medicine, to assist hundred of thousands of Iraqis who have fled their homes in Mosul and Tikrit.
Both cities have been taken over by Sunni insurgents belonging to terror organisation, the Islamic State in Iraq and Syria (Isis), who regard Iraq's Shia majority as "infidels".