That's because he has pinned the success of his new offensive against Muslim extremists on support from Muslim moderates. That includes disciplined, effective combat troops to take and hold ground captured by ISIS.
The logic of Obama's strategy—U.S. airstrikes but no U.S. combat troops in Syria—is that only indigenous forces can do that, and create the opportunity for peace and stability, over the long term. It matches the views of the American public, which has been shocked by gruesome ISIS violence into supporting the new offensive but remains war-weary after more than a decade of conflict in Iraq and Afghanistan.
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The U.S. experience in Iraq shows just how excruciatingly difficult it will be for Obama to walk that line. He followed through on his commitment to end the military conflict President George W. Bush initiated there, arguing that U.S. forces couldn't afford to remain forever. Yet the Iraqi forces trained by Americans for years crumbled in the face of the ISIS advance.
Thus Obama, in his speech at the United Nations, framed his appeal for coalition partners in broad terms. He cast it not merely as a military campaign, but also as a commitment to bring economic opportunity to young Muslims trapped in poverty and alienated from the modern world. Just as he rallied youthful Americans in his two winning presidential campaigns, Obama called it a "generational" task to create robust paths leading away from violent extremism.
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He coupled that imperative with the need to confront what he called the ISIS "network of death," that only understands the language of force. A new round of airstrikes in Syria contains some encouraging signs in the participation of Arab allies, such as Saudi Arabia, Bahrain and the United Arab Emirates. All contain sizable populations of the Sunni Muslims that U.S. officials say must stand up to fellow Sunnis in ISIS if the military campaign can succeed.