President Barack Obama addresses the nation from the Cross Hall in the White House in Washington, Wednesday, Sept. 10, 2014. (Saul Loeb/AP)

THE CIVIL war that erupted in Syria three years ago spawned the vicious, and ambitious, terror force known as the Islamic State, a major threat to the Middle East, Europe and the United States. The rise of the Islamic State, or something like it, was a foreseeable consequence of the Syrian conflict, and of the Obama administration’s failure to come to grips with it. And the beheadings of two American journalists by the Islamic State, after the mass killings and other atrocities by that group across Iraq and Syria, make it plain that any notion of U.S. disengagement from the Middle East is no longer tenable — if it ever was.

President Obama’s reformulation of U.S. policy to accommodate that stubborn reality, articulated in his somber address to the nation Wednesday night, comes late — but not, we hope and trust, too late. There is still time to beat the Islamic State, to restore a unified (if federated) Iraqi state and to bring a measure of peace and humane governance to Syria. As the president convincingly emphasized, none of this can happen without a sustained, long-term engagement by the United States, to include both military force and a major investment of diplomatic capital. As the president also said, for the first time, U.S. strikes on the Islamic State cannot be limited to one side of the porous Iraq-Syria border; American drones and warplanes must pursue the Islamic State wherever it can be found, and that includes its havens in Syria.

We hope Congress welcomes Mr. Obama’s renewed commitment and approves funds he will request to train and equip moderate forces in Syria, as well as to rebuild Kurdish, Sunni and government forces in Iraq. But we also believe that Congress has a duty to go beyond writing the check; it should debate the policy and vote to authorize this mission. Though he believes that he already has the authority to conduct this new campaign, Mr. Obama said Wednesday night that he would welcome congressional action, on the sound principle that “we are strongest as a nation when the president and Congress work together.”

Congressional and public debate are especially necessary to help strengthen those parts of Mr. Obama’s strategy that remain open to question. He cited U.S. policy in Yemen and Somalia as a successful illustration of what’s in store for Iraq and Syria — a one-two punch of U.S. air power with local ground forces. But Somalia is a failed state and Yemen is hardly a healthy one; both remain incubators of dangerous terrorism. We think the president is right that the Islamic State problem cannot and should not be met with an invasion of U.S. ground troops. But it also cannot be solved without U.S. commitment on the ground, both for the military venture — for training, intelligence, and other missions, as Mr. Obama said — and for the larger goal of promoting inclusive governance so as to narrow the political space for terrorism. What will motivate political forces in the region to pursue the necessary compromises is the expectation of durable and multi-layered U.S. support.