An apparently looming American clash with Islamic State in Syria presents a defining test for President Barack Obama's doctrine of often lethal, but limited, no-boots-on-the-ground warfare.
Obama's worldview, cemented when US armies bogged down in Iraq, led him to a brand of war in which drones pound al-Qaeda militants in Pakistan and Yemen and special forces materialize to pursue terror suspects in hostile lands like Libya and Somalia.
But as he contemplates targeting IS militants settling into a new caliphate, he is adamant his core policy of keeping ground troops back home is inviolate.
When Obama sketched his blueprint for flexing US power, at West Point military academy in May, he characteristically positioned himself between quagmire-wary realists and interventionists hooked on the hammer of US military power.
"A strategy that involves invading every country that harbors terrorist networks is naive and unsustainable," Obama said, not knowing his theory would be tested so soon by the IS surge.
He kept faith with his military creed in ongoing air raids against Sunni Islamic State targets in Iraq -- in a war he had previously declared over. US force spurred modest advances by Iraqi and Kurdish forces against IS, also known as ISIL and ISIS, and ethnic Yazidis were spared from genocide.
The White House, despite warlike rhetoric pulsing through Washington, is tempering expectations that a military onslaught in Iraq, let alone Syria, will be decisive.
In Syria, the search for partners is short and unpleasant.
Obama has been loath to arm moderate rebels, leaving anti-IS forces weak. The one potential ally with clout is President Bashar al-Assad. But Obama would have to swallow hard indeed to wage common cause with a man he sees as a war criminal.
Moreover, the track record of America and allies augurs ill for a new Middle East adventure. And US forces sent to pound Syria could be operating in the dark it knows little of the country.
Still, many power players are convinced a new Middle East front is needed, and could work.
Martin Dempsey, chairman of the Joint Chiefs of Staff, says IS can only be defeated if hit in Syria as well as Iraq.
Other foreign policy chieftains warn Obama may have to deal with the devil, amid fears IS zealots armed with Western passports could hit the US homeland.
"The Assad government may be evil -- but it is a lesser evil than ISIS, and a local one," Richard Haass, president of the Council on Foreign Relations, argued in Wednesday's Financial Times.
Though the White House refuses to work with the Syrian leader, he looms large, because he would win if IS is extinguished.