Martial law is when the military helps take over hostile tendencies or chaotic situations of citizens. This does not occur between countries when they are at war. It involves the normal people who live within that country. Usually, martial law happens as a result of citizens invading the capital or government. Sometimes it can be as a result of a disaster or emergency. In most cases, if martial law is used, the national guard is brought in to assist.
The local police department would also assist the national guard. Since the national guard is usually not from that area, they would need help with understanding the situation and the location. The local police would know this and they would help with the situation and giving information.