One notable instance occurred during the Reconstruction era, when the federal government used posse comitatus to enforce federal law in the South. In 1871, for example, President Ulysses S. Grant issued a proclamation authorizing the use of posse comitatus to enforce the Ku Klux Klan Act of 1871, which prohibited the Klan from engaging in violence and intimidation against African Americans.
Another instance of posse comitatus being used in the United States occurred during the Great Depression, when the federal government used posse comitatus to suppress labor unrest. In 1932, for example, President Herbert Hoover issued a proclamation authorizing the use of posse comitatus to quell a strike by coal miners in West Virginia.
In recent years, posse comitatus has been used by the federal government to enforce immigration law. In 2017, for example, President Donald Trump issued a proclamation authorizing the use of posse comitatus to assist the Department of Homeland Security in enforcing federal immigration laws.