For the past two years the Obama administration has resisted entanglement in Syria, but now this grisly civil war has become an unavoidable international problem and a defining moment for American foreign policy.

Having concluded “with high confidence” that the Syrian government carried out a deadly chemical attack on the outskirts of Damascus on Aug. 21, the Obama administration has no choice but to enforce the “red line” the president laid out a year ago. To maintain American credibility — and his own — President Obama has to do so quickly and decisively. He also has to explain to the American people and to the world what is at stake in Syria and how the United States will lead in ending this crisis.

Mr. Obama has understandably viewed any involvement in Syria as a slippery slope to an expensive war that Americans do not want and will not support. Even after President Bashar al-Assad’s use of chemical weapons, his administration has been slow to react, and after much back-and-forth it has decided to punish Mr. Assad — but only in a discrete operation that would not have a direct bearing on the outcome of the civil war, and only, as Mr. Obama suggested on Saturday, if Congress gives its blessing.

The world will not see this as prudence but rather as dithering — reinforcing the perception that the United States is hiding behind its economic woes and, hounded by the ghosts of Iraq, is no longer keen on leading the world. That will embolden America’s adversaries and deject its friends. America could soon find itself alone in standing up to Iran or North Korea, or in pushing back against China and Russia, which have used their veto power on the Security Council to block United Nations authorization for intervention in Syria.