Former defense secretary Robert Gates says President Obama's decision to set a "red line" with respect to Syria's use of chemical weapons was a "serious mistake."

"I always used to say to the president, if you cock the pistol, you've got to be willing to fire it," Gates told the Wall Street Journal in a new interview during his book tour.

Obama set the red line before Syria had used chemical weapons, saying doing so would "change my calculus" on getting involved in that country's civil war.

When Syria actually did it, though, Obama ran into a skeptical Congress that appeared set to deny his request for the use of force. Eventually, a tentative deal was reached under which Syria would turn over its cache of chemical weapons.