WASHINGTON — Russia has sent some of its most modern battle tanks to a new air base in Syria in what U.S. officials said Monday was part of an escalating buildup that could give Moscow its most significant military foothold in the Middle East in decades. Pentagon officials said that the Russian weapons and equipment that had arrived suggested that the Kremlin’s plan is to turn the airfield south of Latakia in western Syria into a major hub that could be used to bring in military supplies for the government of President Bashar Assad. U.S.