[gizmo686]

There is no federal law requiring States to enforce federal drug policy. The inability of the federal government to compel state laws has been a source of tension throughout US history.

For a non-controversial, consider the drinking age in the US. To a first approximation, this was set by Congress in 1984 with the National Minimum Drinking Age Act. However, this law doesn't actually set a drinking age. It witholds federal highway funding from states that refuse to do so.

For a more controversial example, consider the medicaid expansion portion of the Affordable Care Act. The Supreme Court ruled that the federal government could not compell states to implement it, allowing some states to not do so. For completeness, I should point out that the expansion was 100% federally funded to start with, and dropped gradually to 90% by 2020, where it is set to remain indefinitely.

Another major example from US history is the Fugitive Slave Act, which required non-slave states to assist in returning escaped slaves. Some Northern states refused to coorporate with this law and was part of what led to the civil war. (Ironically, this is probably the clearest way that "states' rights" was a cause of the civil war, but I don't think it is what most people have in mind when they say "states" rights".