Carbon border measures (CBMs) are tools that combine climate and trade policies, but they pose new challenges for the World Trade Organization (WTO). The EU and other U.S. allies are cautious about adopting CBMs that may not follow WTO rules. The U.S. under the Trump administration was seen as weakening the WTO, so allies are wary of U.S. policies like CBMs that could go against WTO rules. If the U.S. adopts CBMs that don't align with WTO rules, it could make it harder to restore the WTO's dispute settlement system. The GATT, a key WTO rule, is likely to be violated by CBMs, making it important to consider WTO legal issues when implementing such measures.