At times of crisis, we slip reflexively into finger-pointing mode. President Barack Obama's critics blame the swift fall of major Iraqi cities to Islamist radicals on his decision to withdraw U.S. combat forces from the battered country. His supporters say none of this would be happening had President George W. Bush not destabilized the region by invading Iraq in the first place.

Mitt Romney — say the critics — tried to warn about the revival of radical extremism in the region during the 2012 presidential campaign, and was derided by the president and the news media. Obama — say the defenders — managed to extract U.S. forces from a country where they should never have been.

The debate calls to mind a classic "Doonesbury" comic strip from the 1980s, one I've mentioned before. Conservative Phil Slackmeyer is gloating to his liberal son Mark over Ronald Reagan's victory. Each accuses the other of being insufferable over Watergate, Nixon, Vietnam and, finally, "the Cambodian bloodbath," which the son accuses the father of gloating over. Phil responds, indignantly: "Me? That was you!" Taken aback, Mark asks his father if he's sure. Phil's answer: "Um ... I think so. Whose fault did that turn out to be?"