Under the , courts may be called upon to decide if a state law is invalid because it conflicts with a federal law.

The answer is the supremacy clause.

Explanation:

The Supremacy Clause is established in the Constitution of the United States and states that the federal laws were made according to the Constitution. It also establishes that the different treaties that are made under its authority are considered the "supreme Law of the Land", and in this way, it is most important than any conflicting state laws.

It establishes that state courts, as well as state constitutions, are subordinated to the supreme law. However, the different federal statutes and treaties are considered supreme only in case they do not violate the terms established in the Constitution.    

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