Republican presidential candidate Ben Carson struggled Sunday to attach specifics to his plan to defeat the Islamic State militant group, vividly illustrating the former neurosurgeon's difficulty discussing foreign-policy matters. Speaking on "Fox News Sunday," Carson could not name a specific country or leader he would call to assemble an international coalition to counter the Islamic State, despite being asked three times by host Chris Wallace. "My point being that if we get out there and we really lead and it appears that we're making progress, that all of the Arab states and even the non-Arab states who are, I think,...