A handout image released by the Syrian opposition's Shaam News Network shows a Syrian couple mourning in front of bodies wrapped in shrouds ahead of funerals following what Syrian rebels claim to be a toxic gas attack by pro-government forces in eastern Ghouta, on the outskirts of Damascus on August 21, 2013. The allegation of chemical weapons being used in the heavily-populated areas came on the second day of a mission to Syria by UN inspectors, but the claim, which could not be independently verified, was vehemently denied by the Syrian authorities, who said it was intended to hinder the mission of UN chemical weapons inspectors. (Ammar al-Arbini/AFP/Getty Images.) (AMMAR AL-ARBINI/AFP/Getty Images)

Convinced that Syrian dictator Bashar al-Assad wielded chemical weapons against civilians last week, the Obama administration is considering a military response, according to senior officials. The “large-scale, indiscriminate use” of chemical weapons was a “moral obscenity,” as Secretary of State John F. Kerry said Monday, and some response is needed. But it needs to be part of a larger strategy aimed at influencing the outcome of Syria’s war.

For more than two years, President Obama has avoided crafting such a strategy. When Mr. Assad answered peaceful demonstrations with brutality, the administration did little beyond condemn the violence. Mr. Obama asserted that the dictator was doomed to fall, but Mr. Assad did not take the hint. Assisted by Iran, its terrorist proxy Hezbollah and weapons supplies from Russia, Mr. Assad went to war against his own people, indiscriminately firing missiles into civilian neighborhoods. More than 100,000 people have been killed, with millions more injured or displaced from their homes. Mr. Assad does not seem to be losing the war.

The dangerous outcomes that Mr. Obama worried might be precipitated by U.S. involvement have mostly come about in the absence of such involvement. Syria has become a haven for thousands of fighters affiliated with al-Qaeda. Violence has spread to neighboring states, especially Lebanon and Iraq. U.S. allies Turkey and, especially, Jordan are in danger of being overwhelmed by hundreds of thousands of Syrian refugees. Now, according to Doctors Without Borders and other credible sources, weapons of mass destruction apparently have been used on a scale not seen since Saddam Hussein went after his Kurdish population in 1988, with Mr. Assad seemingly calculating he has little to fear from crossing Mr. Obama’s “red line.”

The U.S. president has been correct from the start that the Syria crisis offered no good options to U.S. policymakers. That has become only more true as the United States has remained aloof. With little support from the West, opposition forces that espouse a multi-sectarian, democratic Syria have found themselves challenged and weakened not only by Mr. Assad’s forces but also by more radical Islamist fighters. This shouldn’t have come as a surprise.

But the fact that Syria offers no perfect outcomes or options does not mean that all possible outcomes are equally undesirable. It remains in the United States’ interest now as two years ago to see more moderate forces prevail. This can’t be achieved with one or two volleys of cruise missiles. It will require patience and commitment.

The United States can’t dictate the outcome in Syria, and it would be foolish to send ground troops in an effort to do so. But by combining military measures with training, weapons supplies and diplomacy, it could exercise considerable influence. The military measures could include destroying forces involved in chemical weapons use and elements of the Syrian air force that have been used to target civilians, as well as helping to carve out a safe zone for rebels and the civilian populations they are seeking to protect.

Such military action should be seen as one component of a policy that finally recognizes a U.S. interest in helping to shape Syria’s future.