At the mouth of the Gulf of Mexico along the Texas-Louisiana border, Cheniere Energy could be just weeks away from breaking ground on the first natural gas exporting facility ever built in the lower 48 states. It’s also where a new fight with echoes of the Keystone pipeline is building, pitting economic development against environmental protection. To Cheniere and its supporters, the 500-plus acre, $10 billion plant represents a boon for the American economy.