Short of an invasion or a civil war I don’t see it happening. The individual states might do it in the case of natural disaster. The state’s ability to do this locally reduces the need to take action like that at the federal level. Perhaps a disaster on the level of the Yellowstone caldera erupting would lead to it but that would be because the affected states would no longer be viable.
The Supreme Court has essentially confined it to situations where civil order has completely broken down. As Wikipedia summarizes: The majority in Ex Parte Milligan stated that “martial rule can never exist when the courts are open” and confined martial law to areas of “military operations, where war really prevails,” and when it was a necessity to provide a substitute for a civil authority that had been overthrown.
From Wiki Martial law in the US:
Martial law in the United States refers to several periods in United States history wherein a region or the United States as whole are placed under the control of a military body. On a federal level, only the president has the power to impose Martial Law. In each state the governor has the right to impose martial law within the borders of the state. In the United States, martial law has been used in a limited number of circumstances, such as directly after a foreign attack, such as Hawaii after the Japanese attack on Pearl Harbor or New Orleans during the Battle of New Orleans; after major disasters, such as the Great Chicago Fire of 1871 or the San Francisco earthquake of 1906; renegade local leaders seeking to avoid arrest, such as Nauvoo, Illinois during the Illinois Mormon War, or Utah during the Utah War; or in response to chaos associated with protests and mob action, such as the 1934 West Coast waterfront strike, or mob actions against the  Freedom Riders.
I don’t know the difference between States’ actions, with the State National Guard (or is the National Guard a federally direct able thing, since they get sent to real wars overseas?), and OP is aware of this and specifically excludes them in his query.
That is, a state National Guard has been used many, many times (natural disasters, eg), and the above listed cases do in fact address OP.
:smack: ETA: What Tripolar said less than half-an hour after OP.\
Although my confusion on the Federal direct orders (ie, “go to Afghanistan”) vs. a Governor’s call-up of a State National Guard persists. No doubt we’ve had a thread or three on that, but can this be addressed, passim, in this thread?
Large scale natural disaster (supervolcano, meteor hit, massive earthquake)
Large scale riots
Large scale terrorist attack
Political coup attempt
Can people file lawsuits with the courts to get martial law overturned? Or are the courts neutered under martial law? Who ends martial law? Does the military end it, the civilian government, the courts?
Individual states maintain National Guard units as part of their state militias.* As a state militia, National Guard units are under the command of the governor of the state. National Guard units are also simultaneously Reserve elements of the U.S. Army (Air National Guard units are Reserve elements of the U.S. Air Force). Under certain circumstances, the President may, by executive order, activate National Guard units as Army Reserve and National Guard (ARNG) units. This is often referred to as “Federalizing.” Once a NG unit has been “Federalized”, it is a unit of the U.S. Army, fully under the command of the President as commander-in-chief of all U.S. armed forces.
Under the legislation establishing the National Guard system, the state and Federal governments share the costs of equipping, training, and maintaining National Guard units while they are in reserve status. The nitty-gritty of the cost-sharing can get pretty convoluted, but basically, when a state governor calls up a National Guard unit, it is fully under his or her command, and that state bears the full cost of the unit’s operations during the state activation. When the President federalizes a unit, it is fully under his command, and the Federal government bears the full cost of the unit’s operations for the duration of the Federal activation.
*I believe Texas actually maintains a completely separate state militia, and AFAIK, is the only state to do so. In all other states, their National Guard contingents are the entirety of their state militias.
If hypothetically all the key people in the federal government were blown up, for example when they were assembled to hear a state of the union address, except for one person that no one liked, who was told to go buy the pizzas, would that trigger martial law?
Yes, government would stop, but there is a chain of succession that perates in the face of a coup, and there is not inherently social collapse.
Martial law isn’t really a concept in the U.S., whatever some may think. Milligan is an acknowledgement of extreme circumstances, not an affirmation of a system wherein the military can take over civilian government.
Thanks for this list … from Wikipedia’s article on “Maryland Defense Force”: “However, as a state defense force, the MDDF is solely under control of the State of Maryland and cannot be federalized or deployed outside the borders of Maryland.”
So in Maryland, the State Militia is separate and distinct from the National Guard …