The United States will declare martial law in extreme emergencies when civilian authority is ineffective or non-existent. This could include natural disasters, insurrections, or civil wars. Martial law is a last resort that involves suspending civil rights and liberties.
When it can be declared
When civilian authority is ineffective or non-existent
When there is an insurrection or civil war
When there is a natural disaster
How it's used Military leaders may create and enforce their own laws, Curfews may be imposed, Public gatherings may be restricted, Military leaders may detain people, and Military leaders may take over local governments.
Who can declare it
The president can declare martial law under extraordinary circumstances
State officials can declare martial law, but their actions must comply with the Constitution