If a state disagrees with a law passed by the Federal Government, should the states be allowed to not follow it?

If a state disagrees with a law passed by the Federal Government, should the states be allowed to not follow it? What if many states disagree with it? Are there any exceptions? Why or why not? Then look at marijuana specifically- If the Federal government deems it illegal (as it does) should a state like Colorado be allowed to make it legal? Then look at Gay marriage- If Alabama prohibits it should they be allowed to refuse marriage licenses to gay couples? why or why not? There are a lot of important concepts here so you should have plenty to write about. Part 2) Propose an amendment to the U.S. Constitution that you think is necessary and describe why you want it in there. Be sure to use full detail here, cite sources, edit, proofread, organize your thoughts,etc. Also be sure to write in third person.