Why would Obama act now, after two years of letting Bashar al-Assad massacre 10,000 of his people? Slate’s Fred Kaplan laid out the rationale insightfully over the weekend. If—we’ll return to this “if”—Assad used chemical weapons, he crossed Obama’s famous/infamous “red line.” In addition, Obama, Kaplan noted, is big on international norms, and one of the biggest international norms going is taking action to prevent the spread of chemical weapons, which has been in place since right after World War I. A failure to act “would erode, perhaps obliterate” the taboo against such weapons. That’s something Obama is absolutely right to take very seriously.

But now, that “if.” We don’t know for sure that it was the regime that used these weapons. We assume it was the regime. But the opposition isn’t exactly a concert of Boy Scout troops. It’s split into many factions, some very anti-American. Maybe the administration has private intelligence fingering the regime. But publicly, it looks pretty strange on its face for the United States to turn down Syria’s offer on inspectors. How could we be moving toward military action without at least going through this motion? The rest of the senior official’s Sunday statement gives a specific reason why it’s too late: because “the evidence available has been significantly corrupted as a result of the regime’s persistent shelling and other international actions over the last five days.”