A new test for Obama's talents: Bringing nations together

President Barack Obama has arrived at the most improbable moment of his tenure. And it poses the most formidable test of his skills so far.

After six years in the Oval Office, the anti-war candidate has become a war president. He never wanted to, and resisted until events in Syria forced his hand.

But he remains the politician who came into office seeking to use his unique, biracial, Kansas-meets-Kenya background to bridge divides internationally as well as domestically. For most of his term, results have been uneven at best. If, in fact, he has a talent for pulling nations together, he needs to show it now.

President Barack Obama speaks at the 69th United Nations General Assembly on Sept. 24, 2014.
Getty Images
President Barack Obama speaks at the 69th United Nations General Assembly on Sept. 24, 2014.

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.

Read MoreObama's ISIS plan still unclear: Pro

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.

Read MoreObama condemns Russia, calls for action against ISIS and Ebola

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.

"Getting Arab countries demonstrably involved is no small achievement," presidential historian Richard Norton Smith told me.

Yet involvement on the ground is a different matter. Saudi Arabia has agreed to serve as a training ground for U.S. forces to prepare moderates of the Free Syrian Army to take on ISIS.

But Arab allies may need to supplement those indigenous Syrian forces with some of their own for the campaign to succeed. Wrangling that sort of effective cooperation will require every bit of the talent for outreach that Obama's supporters have long seen in him.