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."