The corollary is that if we do intervene on the basis that Assad is using chemical weapons, then he might very well start gassing cities — and we won’t be sitting around wondering whether he’s done so or not. When Saddam used chemical weapons against Halabja, there was video of the attack and thousands of casualties. Survivors traveled abroad where they received medical treatment. The evidence was clear and overwhelming.

The evidence we have now is rather less than that. The Syrian opposition has repeatedly claimed that it has been gassed, but these claims have been of doubtful reliability, including allegations that Syria used a chemical weapon that does not exist.

The first lesson of Iraq — and I don’t want to offend your delicate sensibilities here — is that defectors and opposition groups don’t always tell the truth. When President Obama set a red line, he also established a goal for Syrian opposition groups: establish chemical weapons use, get additional U.S. assistance. Pass go, collect $200. Remember the lead-up to Iraq, when an Iraqi defector told Sky News that it was “100 percent guaranteed” that Saddam would use chemical weapons on coalition forces? Defectors. Caveat emptor.