On a rainy May day, Turkish Prime Minister Recep Tayyip Erdogan led two of his closest advisers into the Oval Office for what both sides knew would be a difficult meeting.

It was the first face-to-face between Mr. Erdogan and President Barack Obama in almost a year. Mr. Obama delivered what U.S. officials describe as an unusually blunt message: The U.S. believed Turkey was letting arms and fighters flow into Syria indiscriminately and sometimes to the wrong rebels, including anti-Western jihadists.

Seated at Mr. Erdogan's side was the man at the center of what caused the U.S.'s unease, Hakan Fidan, Turkey's powerful spymaster and a driving force behind its efforts to supply the rebels and topple Syrian President Bashar al-Assad.

In the wake of the Arab Spring uprisings, Mr. Fidan, little known outside of the Middle East, has emerged as a key architect of a Turkish regional-security strategy that has tilted the interests of the longtime U.S. ally in ways sometimes counter to those of the U.S.

"Hakan Fidan is the face of the new Middle East," says James Jeffrey, who recently served as U.S. ambassador in Turkey and Iraq. "We need to work with him because he can get the job done," he says. "But we shouldn't assume he is a knee-jerk friend of the United States, because he is not."