While general understanding of environmental harms has become more geographically sophisticated, environmental-impact assessment (EIA) law has lagged behind. Although nations now understand complex environmental processes and relationships that extend well beyond their borders, EIA law remains trapped in a domestic structure that is ill-prepared to assess harms outside its jurisdiction. By looking at the U.S. environmental assessment of the Keystone XL pipeline, this Note recasts the problem of transboundary environmental harms in EIA using recent, remarkable events. Key assumptions made in the Keystone XL assessment illustrate that the typical domestic structure of EIA law does not allow adequate assessment of transboundary harms and, thus, undermines the entire purpose of the EIA process. After identifying this problem, this Note suggests a number of guidelines for developing binding, cooperative environmental-assessment agreements between states that would bridge that gap and bring transboundary harms into domestic EIA law.