Currently in the news there are discussions about "Sanctuary Cities" who refuse to implement federal laws in regard to illegal immigration, and the state of Colorado, which has legalized recreational marijuana. In that neither of these two decisions is in agreement with federal law—what would the supremacy clause of the Constitution say should happen?

 

Answer:

The supremacy clause would say that federal law would apply and would be enforced. The "Sanctuary Cities" would not be allowed to operate and the federal government would still prosecute illegal immigrants. 

"Sanctuary Cities" who refuse to implement federal laws in regard to illegal immigration, and the state of Colorado, which has legalized recreational marijuana. 

Step-by-step explanation

The Supremacy Clause in the constitution states that the federal constitution and federal law take precedence over state laws and state constitutions. The Supremacy Clause prohibits states from any kind of interference with the federal government's exercise of its constitutional powers.  The US constitution and federal law are the supreme law of the US, so in the case of a conflict between state law and federal law, federal law always wins. 

 The "Sanctuary Cities" would not be allowed to operate and the federal government would still continue to prosecute illegal immigrants. Similarly, in Colorado the federal government considers the possession, use, distribution, and growth of marijuana as a crime. It (federal agents) could and would still continue to arrest and prosecute individuals caught with marijuana.

Additional information: 

https://www.law.cornell.edu/constitution/articlevi

https://www.law.cornell.edu/wex/supremacy_clause

https://constitutioncenter.org/interactive-constitution/interpretation/article-vi/clauses/31

https://uslawessentials.com/supremacy-clause-states-act-federalism/