George Bush has laid the blame for the prolonged war in Afghanistan on NATO allies who didn't pull their weight on the battlefield.
The ex-president blasted critics who claim his administration took its 'eye off the ball' and allowed the conflict to escalate during the invasion of Iraq.
In an interview with CNN, Mr Bush said he assumed allied forces would take up the slack while US troops were busy toppling Saddam Hussein and coping with the ensuing insurgency.
'What happened in Afghanistan was that our NATO allies, some of them, turned out not to be willing to fight,' he said.
'Therefore, our assumption that we had ample troops - US and NATO troops - turned out to be not a true assumption. So we adjusted,' he added.
Mr Bush did not specifically name any countries from NATO, which includes 28 nations in Europe and North America.
After America, the UK, Canada and France have had the most casualties among the 785 non-US troops killed since October 2001 when the former president launched the invasion of Afghanistan in the wake of the 9/11 attacks.
He said he 'felt terrible' that weapons of mass destruction were never found in Iraq, but he still said he thought the war was justified.
'If Saddam Hussein was in power today, the world would be a lot worse off. I believe that a free Iraq will be transformative in the Middle East,' he added.
He insisted he is not trying to shape his legacy with his book. 'I'm trying to provide data points for future historians,' he explained.