"I think I was unprepared for war," Bush told ABC News' Charlie Gibson in an interview airing Monday on "World News."
"In other words, I didn't campaign and say, 'Please vote for me, I'll be able to handle an attack,'" he said. "In other words, I didn't anticipate war. Presidents -- one of the things about the modern presidency is that the unexpected will happen."
Bush, who has been a stalwart defender of the war in Iraq and maintaining U.S. troop presence there, said, in retrospect, the war exceeded his expectations.
"A lot of people put their reputations on the line and said the weapons of mass destruction is a reason to remove Saddam Hussein," Bush said. "It wasn't just people in my administration. A lot of members in Congress, prior to my arrival in Washington, D.C., during the debate on Iraq, a lot of leaders of nations around the world were all looking at the same intelligence.
"I wish the intelligence had been different, I guess," Bush added.
When pressed by Gibson, Bush declined to "speculate" on whether he would still have gone to war if he knew Hussein didn't have weapons of mass destruction.
"That is a do-over that I can't do," Bush said.
Despite failed intelligence and accusations of mismanaging the war, Bush said his decision not to prematurely withdraw troops from Iraq was grounded in his values.
"I listened to a lot of voices, but ultimately, I listened to this voice: I'm not going to let your son die in vain," he said. "I believe we can win. I'm going to do what it takes to win in Iraq."
Bush said that one of his biggest disappointments was the failure to pass a comprehensive bill on immigration reform.
"I firmly believe that the immigration debate really didn't show the true nature of America as a welcoming society," he said. "I fully understand we need to enforce law and enforce borders. But the debate took on a tone that undermined the true greatness of America, which is that we welcome people who want to work hard and support their families."
Bush: Serving as President 'Joyful'
Reflecting on his time in the Oval Office, Bush said that he hopes to be remembered as a president who made difficult decisions in a principled way and "didn't sell his soul for politics."
He also spoke about his role as the "comforter-in-chief."
"The president ends up carrying a lot of people's grief in his soul," he said.
"One of the things about the presidency is you deal with a lot of tragedy -- whether it be hurricanes, or tornadoes, or fires or death -- and you spend time being the comforter-in-chief," Bush said. "But the idea of being able to serve a nation you love is -- has been joyful. In other words, my spirits have never been down. I have been sad, but the spirits are up."
Bush underscored his reliance on his values to guide him through tough moments.
"The thing that's important for me is to get home and look in that mirror and say, 'I did not compromise my principles,'" he said. "And I didn't. I made tough calls. And some presidencies have got a lot of tough decisions to make."
Bush said that he regrets that he was unable to change the partisan tone in Washington -- one that permeated his presidency.
"I didn't go into this naively; I knew it would be tough," he said. "But I also knew that the president has the responsibility to try to elevate the tone. And, frankly, it just didn't work, much as I'd like to have it work."
"9/11 unified the country, and that was a moment where Washington decided to work together," he said. "I think one of the big disappointments of the presidency has been the fact that the tone in Washington got worse, not better."
Nevertheless, Bush said that he felt his administration brought significant change to Washington, with reforms like the No Child Left Behind education policy, and international relief efforts such as the President's Emergency Plan for AIDS Relief and the Malaria Initiative.
President Bush said that his administration responded promptly to the economic crisis.
"When you have the secretary of the treasury and the chairman of the Fed say, 'If we don't act boldly, we could be in a depression greater than the Great Depression,' that's an 'uh-oh' moment," he said.
Through government action such as the Troubled Assets Relief Program, Bush insisted that the government has taken the necessary first steps towards economic recovery.
"Slowly but surely, the system is becoming unthawed, and it's going to take time for the system to become unthawed," he said. "What the American people have got to know is we've taken the steps to unthaw it, which is the first step to recovery.
"The American people got to know that we will safeguard the system," he said. "I mean, we're in. And if we need to be in more, we will."
While many have cast blame on the Bush administration for mishandling the economy, the president said he did not have feelings of guilt for the financial collapse.
"You know, 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, before I arrived in president," Bush said. "And when people review the history of this administration, people will say that this administration tried hard to get a regulator. And there will be a lot of analysis of why that didn't happen. I suspect people will find a lot of it didn't happen for pure political reasons."
Bush on President-Elect Obama's 'Smooth Transition'
President Bush said that he encouraged President-Elect Barack Obama to become involved in the planning and implementation of the economic stimulus plan, personally calling the Obama for a briefing on the government's decision to infuse money into Citigroup.
"This is a very unique period in American history where a new president is coming in, where we are fighting a two-front war against terrorists and, at the same time, dealing with a very difficult economic situation," he said. "And the more we can work together, the better off our country will be."
Obama has assumed a leading role on the economy, announcing key members of his economic team in three consecutive press conferences last week in Chicago, despite assurances that there that there is only "one president at a time."
But Bush insisted that he did not find the president-elect's role intrusive.
"I don't feel any intrusion whatsoever&Our administration still will be making the decisions necessary until he becomes the president."
Asked if Obama's election was in any way a repudiation of his administration, Bush saw more nuance.
"I think it was a repudiation of Republicans," he said. "And I'm sure some people voted for Barack Obama because of me. I think most people voted for Barack Obama because they decided they wanted him to be in their living room for the next four years explaining policy."
He also said that Republican presidential nominee Sen. John McCain faced a "tough headwind" in the campaign, particularly in terms of the economic climate and the perception of the Republican party.
Bush praised the Obama campaign's organization and message, and when asked what his parting relationship will be with President-elect Obama, Bush said that he will reach out to the incoming president.
"One of my parting words to him will be: 'If I can help you, let me know,'" Bush said.
Bush Looks Forward to Life Out Of Spotlight
President and Mrs. Bush agreed that said that they've had enough of the limelight and are looking forward to living a "normal daily life."
"It's going to be an interesting adjustment. We'll adjust. We got each other, we've got our kids, we've got fabulous friends in Texas," Bush said.
"I'm going to have a lot of time to think," he added. "My day is going to go from getting up early-early, and being at the Oval Office at 6:45 a.m., and having a lot to do when you get there, to waking up at 6:45 a.m., getting Momma the coffee -- and kind of wandering around trying -- 'What's next, boss?'"
Bush said he plans to write a book and to continue serving the country with his wife through an institute for policy and library at the Southern Methodist University.
As they prepare to say goodbye to the White House, the first lady said she thinks the country is thankful for her husband's leadership.
"I think they think he's somebody that kept them safe for eight years," she said. "And I hear that all the time, people thanking me, telling me to thank him."