THREE MAJOR U.S. allies — Britain, France and Israel — have now concluded that the Syrian regime of Bashar al-Assad has very likely used chemical weapons, not once but on multiple occasions. This would cross a “red line” drawn by President Obama. An Israeli general said Tuesday that “the regime used lethal chemical weapons against gunmen in a series of incidents in recent months,” including a March 19 attack near Aleppo where a “sarin-type” substance was employed. The British and French governments reported to U.N. Secretary General Ban Ki-moon that they had corroborating evidence, including soil samples, of chemical-weapon use in three instances since December.

Though his policy on Syria has been weak and muddled, Mr. Obama has been very clear that the United States “will not tolerate the use of chemical weapons against the Syrian people,” as he put it last month. He has said that such use would be a “game-changer.”

Yet the administration now declines to join the analysis of its close allies. Perhaps it is the case, as White House spokesman Jay Carney said Monday, that “conclusive evidence” is still lacking. Administration officials point out that the British, French and Israelis are not saying they are certain. No one wishes to repeat the mistake of intervening in Iraq on the basis of faulty intelligence.

But another danger is that the Assad regime will conclude that Mr. Obama’s warnings were never serious. Many analysts believe the relatively limited use of chemical weapons until now was intended to test international reaction. If there is no response, Damascus may decide that it is free to use its chemicals on a larger scale.

Mr. Assad suggested something like that this week in a meeting with a Lebanese delegation. According to a Lebanese newspaper report, he said, “The Americans have been pragmatic from the very beginning and never pursued any course to its logical conclusion. They would eventually side with the victor.”

In truth, Mr. Obama has been inching toward more decisive action. At a meeting of opposition supporters last weekend, Secretary of State John F. Kerry announced a doubling of U.S. aid for the opposition to $250 million, including direct funding for rebel forces.

Yet U.S. support for the rebels remains far below what would be necessary to accelerate the downfall of the Assad regime. Syrians are furious at the United States and increasingly supportive of an al-Qaeda militia that has won a string of battlefield victories. If Mr. Obama waffles or retreats on the one clear red line he drew, U.S. credibility across the region will be severely damaged.

With Syria blocking a U.N. investigation and few assets on the ground, it can be difficult to determine what happened in the reported chemical-weapons attacks. But it is important that the United States reach a conclusion, and soon.