What law or policy is a criminal code that applies to Service members and in time of declared war or a contingency operation persons serving with or accompanying an armed force in the field?

The law you are referring to is the Uniform Code of Military Justice (UCMJ). The UCMJ applies to all members of the armed forces, including those serving in time of war or contingency operations. It outlines the criminal code and procedures for service members, ensuring discipline and accountability within the military.