Saudi Defence Minister Mohamed bin Salman (L) and Crown Prince and Interior Minister Mohammed bin Nayef attend the 136th Gulf Cooperation Council (GCC) summit, in the Saudi capital Riyadh, on December 9, 2015. Gulf monarchs began arriving in Saudi Arabia for an annual summit, facing challenges including plunging oil revenues, the war in Yemen, pressure for peace in Syria and signs of regional divisions. AFP PHOTO / FAYEZ NURELDINE / AFP / FAYEZ NURELDINE        (Photo credit should read FAYEZ NURELDINE/AFP/Getty Images)