Two-time Secretary of Defense Donald Rumsfeld famously articulated a three-tiered framework of human knowledge. First, there are “known knowns,” he explained. These are the things “we know we know.” Second, there are “known unknowns,” the things we know that “we do not know.” And third, there are “unknown unknowns,” the things “we don’t know [what] we don’t know.”

He was right. This is an insightful framework for categorizing human knowledge — which is often imperfect and incomplete.

Yet Rumsfeld’s most famous endeavor — the Iraq War — was a profound violation of the principle that people should recognize the limits of their own knowledge. While advocating for war against Iraq, Rumsfeld was far too confident that Iraq had weapons of mass destruction. He thought it was a known known — a certainty — when it was really a known unknown. While we knew that Saddam Hussein had sought weapons of mass destruction, we didn’t know, at the time of the invasion, whether or not he actually had them. 

We now know he didn’t.

Compounding this error in judgment, Rumsfeld was far too confident in thinking the war in Iraq would be successful. He was convinced that after defeating Saddam’s army, the U.S. could turn Iraq into a stable democracy.

We now know we couldn’t. 

Despite Rumsfeld’s overconfident assertion that “I don’t do quagmires,” Iraq is — still — not the vibrant democracy he and others in the Bush administration predicted it would become.

But Rumsfeld’s contradictions don’t stop there. On the one hand, Rumsfeld was brilliant, dedicated and farsighted. His work transitioning the military away from a Cold War posture and toward addressing modern asymmetrical threats involving terrorism and new technologies was necessary and important. 

On the other hand, Rumsfeld was far too rigid in his own views to build the wide coalitions of allies — domestic and foreign — he needed to accomplish many of his objectives. And his gratuitous antagonism toward the U.S. press corps was an unforced error in a democratic society where perceptions often matter more than reality.

Rumsfeld’s great strengths were thus often marginalized by his profound weaknesses.

Rumsfeld died last week at the age of 88. By the end of his life, he was a known known in American politics — his career in government began in 1962 when he was elected to Congress (at age 30) and ended in 2006 when he left the Pentagon. Yet he will be defined by one lasting open question: How could someone so attuned to the limits of human knowledge be so overconfident in his pre-war assessments of Iraq? 

This question — a known unknown — is unlikely to ever be answered definitively. “Freedom,” Rumsfeld once said, “is untidy.” So, too, will be his legacy.

William Cooper is an attorney who has written for The Wall Street Journal, New York Daily News, Baltimore Sun, Orlando Sentinel and USA Today, among others.