The Middle East would be "far more perilous than it is today" with Saddam Hussein in power, Rumsfeld wrote in his new book, "Known and Unknown: A Memoir" -- slated to be released next week.
While the book's contents have been a tightly guarded secret, some revelations have already leaked out.
The book covers the span of an extraordinary career in Washington and beyond, as Rumsfeld himself recorded in real-time in thousands of pages of documents being released in conjunction with the book.
Elected to Congress from Illinois at age 30, he went on to serve in top posts in the Nixon and Ford administrations -- including Ford's White House chief of staff -- and was the nation's youngest ever as well as oldest ever Defense secretary, tenures separated by 24 years in the private sector.
In the memoir, Rumsfeld recounted a one-on-one meeting with President George W. Bush, who first asked for Iraq war plans just two weeks after the Sept. 11 attack -- before the U.S. invasion of Afghanistan.
Watch Diane Sawyer's exclusive interview with Donald Rumsfeld Monday on "World News" at 6:30 p.m. ET and "Nightline" at 11:35 p.m. ET, and watch Rumsfeld live with George Stephanopoulos on "Good Morning America" on Tuesday at 7 a.m. ET.
As defense secretary, Rumsfeld played a key role in the war on terror – often referred to as one of the architects of the war in Iraq.
In responding to criticism that he didn't authorize enough troops to fight in the war, he said that commanders never sent him a request for more forces in 2003.
But he wrote that, "in retrospect, there may have been times when more troops could have helped."
Abu Ghraib Regrets and Resignation
However, if there was one thing he regrets, it's not quitting after the Abu Ghraib detainee abuse scandal.
Graphic photos showing physical and sexual humiliation of detainees at the prison in Iraq by U.S. soldiers touched off worldwide outage.
"Abu Ghraib and its follow-on effects, including the continued drum-beat of 'torture' maintained by partisan critics of the war and the president, became a damaging distraction," he wrote. "More than anything else I have failed to do, and even amid my pride in the many important things we did accomplish, I regret that I did not leave at that point."
Rumsfeld wrote President George W. Bush two letters of resignation but both were rejected.
He was dismissed by President Bush after Democrats took over Congress in Nov. 2006, after serving nearly six years as Defense Secretary in his second stint in the job he also held in the Ford administration.
Bush, National Security Team Criticism
While serving under Bush, Rumsfeld was critical of the president's management style but wrote that Bush was "a far more formidable president than his popular image."
Rumsfeld also criticizes other members of the Bush national security team -- Condoleezza Rice, Colin Powell and, especially, Iraq administrator Paul Bremer.
"There were far too many hands on the steering wheel, which, in my view, was a formula for running the truck into a ditch," he wrote.
As for Hillary Clinton, Joe Biden and other critics who had at first supported the Iraq war, Rumsfeld wrote: "You wouldn't want to be in a foxhole with them."
ABC News' Rick Klein and Eric Avram contributed to this report.