In this photo released by the Oman News Agency, ONA, James Mattis, U.S. Defense Secretary Jim Mattis, right, shakes hands with Omani Brigadier Saleh bin Ahmed al-Hinai, Head of the Oman Military Protocols and Public Relations, at Seeb Air Base, northwest of Muscat, Oman, Sunday, March 11, 2018. Mattis on Sunday, warned the Syrian government not to use chemical weapons in its civil war. Mattis said Russia, which intervened militarily in Syria to support the Assad government, could be complicit in