Before you read this article it is important that you know what posse comitatus is. It is a rule of law that the military cannot be used to enforce civil law except under explicit circumstances. Wikipedia provides the following information:
The original provision was enacted as Section 15 of chapter 263, of the Acts of the 2nd session of the 45th Congress.
Sec. 15. From and after the passage of this act it shall not be lawful to employ any part of the Army of the United States, as a posse comitatus, or otherwise, for the purpose of executing the laws, except in such cases and under such circumstances as such employment of said force may be expressly authorized by the Constitution or by act of Congress ; and no money appropriated by this act shall be used to pay any of the expenses incurred in the employment of any troops in violation of this section and any person willfully violating the provisions of this section shall be deemed guilty of a misdemeanor and on conviction thereof shall be punished by fine not exceeding ten thousand dollars or imprisonment not exceeding two years or by both such fine and imprisonment
The act has since been modified to allow use of the military in certain instances of disaster or terrorism. You can read the whole Wikipedia article HERE.