Whether international organizations constrain great powers or are controlled by them %or allow great powers to exploit other countries is at the core of international politics. If international organizations are to exert a constraint, their international rules have to be impartial and immune from influence --this is a fundamental requirement for justice. But is international law really just? Are international courts within international organizations blind to power politics? Existing evidence is mixed ---arguably because of inferential challenges. I adjudicate between these positions by presenting new causal evidence. I leverage a natural experiment in the World Trade Organization's Appellate Body (AB) in which international judges ---the members of the AB--- are randomly assigned to cases. Panel composition is consequential for countries, who see their claims more likely to be accepted when they face a panel with a co-national judge. However, further tests demonstrates this is driven by the US. Additional tests rule out alternative explanations. These findings have implications for our understanding of international institutions and the role of US in international politics.