A single cross-border pipeline project has been the epicenter of environmental litigation for the last decade—and it is not over yet. For years, TransCanada Keystone Pipeline, LP and TC Energy have sought to construct and maintain a segment of the Keystone pipeline between the United States and Canada to connect existing pipeline infrastructure and transport crude oil. To do so, the company must first apply and be approved for a permit. Between 2008 and 2012, President Obama twice denied TransCanada Keystone Pipeline and TC Energy’s applications. Then, in 2017 and again in 2019, President Trump unilaterally invited TC Energy’s application and approved the permit. Plaintiffs challenged the 2017 permit in a separate case. This case centers up...