Which part of the constitution lets national laws take precedence over state laws if the two are in conflict?

Article IV of the US constitution has the Supremacy Clause that provides that federal law is the supreme law of the land. This clause requires all judges to follow the federal government's laws, treaties, and constitution on matters and issues under the federal government's indirect or direct control. The doctrine of preemption within the clause assert the supremacy of federal law over state law whenever the two are in conflict.