Evidence continues to mount that a deadly chemical weapons attack has taken place in Syria, causing hundreds and perhaps thousands of casualties.
Despite protestations of innocence from the government of President Bashar al-Assad, the regime looks culpable: It's unlikely that the Syrian rebels possess such weapons, while the Syrian government admits to having them and very likely has used them before.
Many governments, including Syria's patron, Russia, have called on Mr. Assad to permit international chemical weapons investigators, already in Syria, to examine the area. So has UN Secretary-General Ban Ki-moon. This would be the best way to determine conclusively what has happened. So far, Syria has not agreed.
The rebels themselves may be able to smuggle out hard evidence – such as tissue samples from the casualties or spent rockets. But because the affected area just outside Damascus is the scene of ongoing fighting, rebels may have a hard time extracting evidence for analysis.
Indisputable proof would obliterate the "red line" President Obama drew last summer in warning the Syrian government not to resort to chemical attacks. It would raise the stakes dramatically: The world hasn't seen the use of chemical weapons in warfare since Saddam Hussein employed them against Iran and his own people in the 1980s.
After millions of combatants in World War I were killed or wounded by chemical weapons, they became a taboo on battlefields. A breech of that nearly century-old ban would require a tough response. Both Mr. Obama and the British and French foreign ministers said as much this week.
Even US congressional doves now seem ready to back a stronger US response, which could include missile attacks from US ships or planes on Syrian military targets.
Why would Assad risk an international military response? His forces have turned the tide in the civil war and seem to be regaining ground. And a chemical attack with international inspectors already in Syria would seem ill advised. Perhaps he was counting on just these conditions to act as a smoke screen for a bold move.
The exit of the Assad regime would be the best outcome for the region and especially for Syrians themselves. Assad is unlikely to crush the rebels. Instead he's headed toward control of a rump Syrian state with its economy in ruins and its people suffering. He will be highly dependent on Russia and especially Iran to prop him up.
With Assad in only partial control, rebel-held areas of Syria could devolve into lawless fiefdoms. Both regions would contain forces hostile to neighboring Israel, the closest US ally in the region. Syrian moderates would likely continue to flee to Jordan and elsewhere leaving the country even more radicalized.
The US has promised to arm the rebels, but according to their spokespeople no such arms have arrived. A first step would be for the United States to follow through on this commitment.
Now is also the time for the US administration to work closely with America's allies to fashion a measured but ever-escalating response until Assad is forced out. It is a strategy not without risks, but the stakes now are too high for the world to ignore a crossing of the red line.