What is the law applied by the military during emergency?

Answer 1

martial law

NO. The military itself does NOT get to apply it's law to civilians, nor engage in civilian law enforcement activities, per the Posse Comitatas Act. That is, the military may not engage in law enforcement activities on US soil where civilian law applies. Period. No exception. Only in special areas (generally military bases) designated by Congress as permanent military installations can military law be applied to civilians.

In an "emergency", certain civilian leaders (mostly, the Executive leader of a jurisdiction) may go through a specified process to declare "Martial Law", which is a special subsection of civilian law. The enforcement of the sections of Martial Law is still NOT legally allowed to be done by members of the US Military.

The fuzzy part is the National Guard. The NG occupies a strange space in the US system: normally, it is defined as the civilian militia of a State, and, while its members are subject to Military Law while on duty, the NG as a whole is responsible to the State Governor (and, ultimately, the State Legislature). As such, the Governor (when invoking Martial Law) can effectively deputize all members of the NG as Law Enforcement Officers of that state. Thus, a NG member in a place where Martial Law has been declared, has the power to detain and arrest civilians. Infractions are still of civilian law, and cases tried in civilian courts.

The news and TV shows gloss over this all the time, but it's a very important distinction. Let's spin a typical scenario of a disaster: A huge hurricane runs through a state, causing widespread disaster. Afterwards, the police departments are devastated and unable to control looting. The Governor then declares martial law in specific areas (usually not statewide) under his jurisdiction. The State's National Guard is deployed, but is overwhelmed. The Governor requests aid from nearby states, which send their NG to help. This still isn't enough, and the Governor requests federal aid. US Army troops are deployed.

In the above scenario, presuming that the problem is not a general rebellion (i.e. merely a disaster, not the start of a civil war), the original State's NG is empowered automatically as Law Enforcement. If there is a pre-existing agreement between states (or, the Legislature manages to pass one during the emergency), then the Governor can recognize the other states' NG as valid law enforcement too. HOWEVER, the US Army cannot be employed in a law enforcement role, no matter what the Governor wishes. The US Army can still help with rescue and all sorts of other roles, but law enforcement duties cannot be performed by them.

The line is very clear, and very old. No active-duty member of the US Armed Forces may engage in civilian law enforcement activities in a civilian area, with one exception: specifically designed military law enforcement personnel may enforce federal civilian law wherever it applies. No military member may ever enforce military law in a civilian area, under any circumstances. They may always enforce military law in areas where it is specifically designated as applying (e.g. military bases), and they may arrest and detain for violations of civilian law in a place normally enforcing military law, but such violators must be turned over to civilian authorities for prosecution.