From engaging in sexual relations with an intern to letting the Vietnam War escalate, U.S. presidents have been blamed for some egregious errors. So who had the worst blunder? President James Buchanan, for failing to avert the Civil War, according to a survey of presidential historians organized by the University of Louisville's McConnell Center.

The survey's top 10 presidential blunders were announced Saturday during a President's Day weekend conference called "Presidential Moments."

"We can probably learn just as much -- or maybe even more -- by looking at the mistakes rather than looking at why they were great," said political scientist and McConnell Center Director Gary Gregg.

Scholars who participated said Buchanan did not do enough to oppose efforts by Southern states to secede from the Union before the Civil War.

The second worst mistake, the survey found, was Andrew Johnson's decision just after the Civil War to side with Southern whites and oppose improvements in justice for Southern blacks beyond abolishing slavery.

"We continue to pay" for Johnson's errors, wrote Michael Les Benedict, an Ohio State University history professor emeritus.

Lyndon Johnson earned the No. 3 spot by allowing the Vietnam War to intensify, Gregg said.

Where does Bill Clinton's Monica Lewinsky scandal rank? Many scholars said it belonged at No. 10, saying that it probably affected Clinton's presidency more than it did American history and the public.