States have their own sovereignty due to the United States Constitution. The United States Constitution provides that the states can govern themselves, which allows the States to enact their owns laws. Many state and federal laws can overlap as well. However, when a federal law and state law directly conflict with each other, then the federal law will supersede the state law and the federal law will govern. This is because the Supremacy Clause of the United States Constitution states that the federal law is the “supreme” law of the land. Federal law can also govern one area of law and exclude States from enacting any laws pertaining to this area of law. Generally speaking, a federal crime normally has a more severe punishment and will often result in higher fines and longer imprisonment periods.
Usually, most crimes are prosecuted at the State level. However, there are certain circumstances where the federal prosecutors will choose to prosecute a defendant rather than defer to the State prosecutors to prosecute the crime. The areas that the Federal prosecutors will often prosecute are national security, taxes, military, post office and interstate business.
If a defendant meets the elements of both crimes, a defendant can be charge and prosecuted for both state and federal criminal charges. Double jeopardy only applies when the same entity tries a defendant twice for a charge. Since the federal government and the state government would constitute two different entities – this would not be considered double jeopardy. The same goes for another state – two states can prosecute a defendant for the same crime if the elements of the crime are met beyond a reasonable doubt.