In his first days in office, President Trump reversed the government’s position on a highly contentious energy project, reviving the Keystone XL , a pipeline that would link oil producers in Canada and North Dakota with refiners and export terminals on the Gulf Coast. The pipeline has long been at the center of a struggle pitting environmentalists against advocates of energy independence and economic growth. President Barack Obama rejected the project in late 2015, saying it would be antithetical to the United States’ leadership in curbing reliance on carbon fuels.