A long-developing proxy clash in Syria between Russia and Turkey is “very much moving into crisis mode,” a senior American diplomat said amid reports that Turkish troops were attacked in an airstrike. “Oh my gosh,” Kay Bailey Hutchison, the U.S. ambassador to NATO, exclaimed Thursday when she learned from journalists that more than 20 Turkish service members reportedly have been killed. “Let me say this because I just learned this: Of course, everything is on the table. This is a new development. This is a big development.” Turkish President Recep Tayyip Erdogan called an emergency meeting of his national security...