Ten years on from the start of the war, the former prime minister drew parallels with the Syrian crisis to claimed there was a "price for standing aside as well as intervening".
It comes ahead of a BBC Panorama documentary claiming that intelligence used to justify the invasion of Iraq 10 years ago was based on "fabrication" and "wishful thinking".
The programme, to be broadcast on Monday evening, suggests that US and UK security services relied on several pieces
of questionable information, while dismissing others that were contradictory.
It tells how claims from a few sources that Iraq was manufacturing Weapons of Mass Destruction spiralled into apparently sound intelligence used to justify the war.
The war, which started on March 20, 2003, lasted over six years, claimed the lives of 179 UK personnel, more than 100,000 Iraqis, and and cost more than £9 billion.
Britain ended combat operations in 2009 but a decade on the war remains unfinished business, with ongoing questions about the legality of the invasion, and the conduct of British troops.
In an article for The Sun, Blair wrote: "Ten years on from Iraq, as well as looking at the toughness of the struggle our forces faced there, we should think about what would have happened if we had backed away from removing Saddam.
"Look at what Assad in Syria is doing to his people following the Arab upheavals and the effect on the whole of the Middle East, then think what Saddam would be doing in Iraq and the impact it would have.
"Yes, Iraq still has huge problems. The terrorists still commit their acts of terror. But that is not the whole picture."
He claimed progress had been made in Iraq that would have been impossible under Sadam, who he branded a "brutal disaster for the Iraqi people."
Paying tribute to the Armed Forces, he added: "Their commitment and sacrifice, along with many others, including our American allies and many brave Iraqis, is the reason Iraq has a chance today.
"In a region of turmoil where our interests and security remain linked, that is vital."