Kurdish Peshmerga fighters fire at Baretle village, which is controlled by the Islamic State, in Khazir, on the edge of Mosul September 8, 2014. The Kurdish fighters are firing from an area they had retaken from the Islamic State, on Bashiqah mountain. (Ahmed Jadallah/Reuters)

PRESIDENT OBAMA and top aides are now employing the “D” word — “destroy” — to describe U.S. objectives regarding the fanatical Middle Eastern force known as the Islamic State. Already, the group has seized far more of Iraq and Syria than is compatible with the safety and human rights of the people living there, and its sights are set on further destabilization in Jordan, Saudi Arabia and Kurdistan, as well as terror attacks in Europe and, if it’s capable of them, the United States. The two Americans butchered by the Islamic State will not be the last if the group’s leaders have their way. This murderous terrorist army, whose scarily effective global recruitment matches its global ambitions, can be neither contained nor “managed,” as the president implied in some of his more hesitant previous comments.

We are glad that the president has come around to a more sober view. But if he is truly committed to the group’s defeat, certain things must follow from that determination. First, the objective — victory — must determine the strategy, tactics and schedule. Heretofore, Mr. Obama has had an unfortunate tendency to do things the other way round: to view military conflict as something to be carried out according to a schedule, whereby U.S. forces must be withdrawn on a particular date, whether their goals were lastingly achieved or not. He has described his country as tired of war, and, in multiple instances, ruled out certain means — ground forces especially — before anyone has even asked for them. He wishfully mused that the tide of war had “receded.” Now, if Mr. Obama believes that the destruction of the Islamic State is essential to U.S. security, he must commit to that goal and fashion whatever strategy is necessary to achieve it.

Second, he must explain its necessity to the American people and win the support of their representatives in Congress. Administration officials concede that it could take years to root the Islamic State out of the territory it holds. They know also that the goal cannot be achieved without expense and risk to U.S. service members. Mr. Obama has said he has no intention to order an Iraq-style invasion and occupation. But he already has dispatched more than 1,000 fighters to Iraq, and more will surely be required to train, target airstrikes and accomplish other jobs. He would not bomb Syria’s chemical weapons arsenal without congressional authorization; surely such buy-in is all the more important for a multi-year campaign against a terrorist organization operating in multiple nations. We believe the mission he is laying out is essential; we also believe the nation should not embark on it without a commitment to see it through.

Mr. Obama begins to make his case to Congress and the nation Wednesday. In seeking their support for what may be the first long-term overseas war to begin entirely on his watch, the president should be utterly forthright about the risks of inaction but also about the potential costs of action. Only a clear-eyed president, backed by an informed people and their representatives, can lead the world in this crucial mission.