Starting before WWII, the town of Phenix City, AL had been a hotbed of organized crime activity. This was driven by the proximity to the US Army's Fort Benning, which is located just across the river in Columbus, GA. Mostly, it seems that the town existed to provide illegal services (gambling and prostitution) to the off-duty soldiers. Since Fort Benning was and is a primary training base, the constant rotation of troops in and out meant that there was always fresh meat for the criminal operations to seduce and entrap. The military authorities struggled to limit the effect on the base, but the city government seemed completely complicit in the illegal activities.

In 1954, a Phenix city resident, Albert Patterson, seemed to be poised to be elected as the Attorney General for the state of Alabama. His platform for the election was cleaning up Phenix City, which was an embarrassment to the state as a whole. (Place this in the general context of 1954, with Brown v. the Board of Education happening and the state of race relations in the state, and you can get some sense of just how bad things were.)

When Patterson was murdered/assassinated at his home in Phenix City, Governor Persons got the state government directly involved. This lead to the deployment of the National Guard, and on July 22, 1954, martial law was declared for the city by Governor Persons, in coordination with President Eisenhower and FBI Director Hoover. It lasted for about six months, until Jan. 17, 1955.

By that time, about 150 people had been indicted for various crimes up to and including murder, and the vast majority of them were eventually convicted.

Depending on how you define martial law, you might point to more recent incidents of protests/riots. But the story of Phenix City is the only time since Reconstruction that a locally elected government had to be forcibly removed by the Army, so I think it's worth notice.

You can read about this in great detail in the book Phenix City: The Wickedest City in America by Strickland and Wortsman. You might enjoy the more novelized account presented in The Tragedy and Triumph of Phenix City, Alabama