"I wish the intelligence had been different."
But Bush refused to say whether he would have ordered the March 2003 invasion if he had known Iraqi's former leader, Saddam Hussein, did not have weapons of mass destruction, calling it "an interesting question".
"That is a do-over that I can't do. It's hard for me to speculate," said Bush, who declared as recently as last week that Saddam's ouster was "the right decision then - and it is the right decision today".
Hundreds of thousands of Iraqi civilians and more than 4,200 US troops have died in Iraq since Bush launched the war.
A months-long public campaign centered on the grounds - later proved false - that Saddam possessed weapons of mass destruction triggered the war.
"A lot of people put their reputations on the line and said the weapons of mass destruction is a reason to remove Saddam Hussein. It wasn't just people in my administration," Bush told ABC.
Asked what his greatest accomplishment was, Bush replied: "I keep recognising we're in a war against ideological thugs and keeping America safe."
| Bush said he was sorry for the economic woes that have seen US firms failing [EPA]
Asked what he was most unprepared for when he took office in January 2001, Bush replied: "I think I was unprepared for war. In other words, I didn't campaign and say, 'Please vote for me, I'll be able to handle an attack.'"
"I didn't anticipate war. Presidents - one of the things about the modern presidency is that the unexpected will happen," he said.
Bush, whose administration recently accepted a formal timeline for withdrawing from Iraq, also stood fast behind his refusal for years to set a pull-out timetable.
"It would have compromised the principle that when you put kids into harm's way, you go in to win," he said.
'I am sorry'
Asked about the global economic crisis, Bush declared "I'm sorry it's happening, of course," but rejected any effort to blame his administration for inaction in the face of growing concerns.
"I'm the president during this period of time, but I think when the history of this period is written, people will realize a lot of the decisions that were made on Wall Street took place over a decade or so," he said.
Bush also described much of his time in office as "joyful" even though "the president ends up carrying a lot of people's grief in his soul during a presidency".
"I don't feel joyful when somebody loses their life, nor do I feel joyful when somebody loses a job. That concerns me," he said.
"But the idea of being able to serve a nation you love has been joyful."
Asked what Americans would say when he left office, Bush replied: "I hope they feel that this is a guy that came, didn't sell his soul for politics, had to make some tough decisions, and did so in a principled way. I will leave the presidency with my head held high."