This is the supremacy clause as stated within the federal constitution. It is the tool used by those who believe that the federal government has unlimited power across all states, cities, and jurisdiction. What those people don't realize is that a federal law's supremacy status conditional as long as it is "in pursuance thereof" (constitutional) or enforcing an existing treaty signed by the federal government. When it does not fit into the supremacy clause's set of conditions then that federal law is no longer the supreme law of the land. At this point, any state, city, or country law becomes the "supreme law" over that governing authority's jurisdiction which effectively nullifies federal law within that jurisdiction (this assumes that there is no state law that overides city or county law). While the supremacy clause may appear to give the federal government the right to override any state, city, or country law it actually gives way more power to nullify federal law than the tenth amendment itself. The tenth amendment only applies to state governments and their jurisdictions but the supremacy clause gives all internal governments that are capable of making and enforcing laws the power to nullify federal law when that law does not follow the conditions set out in the supremacy clause.