WASHINGTON — If President Obama hoped that the danger of chemical warfare in the Middle East receded when Syria gave up tons of poison gas, mounting evidence that toxic weapons remain in the strife-torn country could once again force him to decide just how far he is willing to go to enforce his famous “red line.” The discovery of traces of ricin and sarin in Syria, combined with the use of chlorine as a makeshift weapon in the country’s grinding civil war, undercut what Mr. Obama had viewed as a signal triumph of his foreign policy, the destruction of President Bashar al-Assad ’s chemical arsenal. But Mr. Obama appears no more eager to use military force against Mr. Assad’s government today than he was in 2013 when he abruptly called off a threatened airstrike in exchange for a Russian-brokered agreement in which Syria voluntarily gave up its chemical weapons. […]