The U.S. Supreme Court has formally requested the US government’s opinion in a case involving Spain and European solar investors. Specifically, Spain is appealing against the enforcement of these awards in the US on the basis that intra-EU arbitration proceedings under the Energy Charter Treaty are invalid due to conflict with European Union law, particularly following the decision of the European Court of Justice in Achmea. The opinion of the US government could have significant implications for the treatment of intra-EU arbitral awards in the US.