From Wikipedia: Ku Klux Klan Act

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The Enforcement Act of 1871 (17 Stat. 13), also known as the Ku Klux Klan Act, Third Enforcement Act,[1] Third Ku Klux Klan Act,[2] Civil Rights Act of 1871, or Force Act of 1871,[3] is an Act of the United States Congress which empowered the President to suspend the writ of habeas corpus to combat the Ku Klux Klan (KKK) and other terrorist organizations. The act was passed by the 42nd United States Congress and signed into law by United States President Ulysses S. Grant on April 20, 1871. The act was the last of three Enforcement Acts passed by the United States Congress from 1870 to 1871 during the Reconstruction Era to combat attacks upon the suffrage rights of African Americans. The statute has been subject to only minor changes since then, but has been the subject of voluminous interpretation by courts.

This legislation was asked for by President Grant and passed within one month of when he sent the request to Congress. Grant's request was a result of the reports he was receiving of widespread racial threats in the Deep South, particularly in South Carolina. He felt that he needed to have his authority broadened before he could effectively intervene. After the act's passage, the president had the power for the first time to both suppress state disorders on his own initiative and to suspend the right of habeas corpus. Grant did not hesitate to use this authority on numerous occasions during his presidency, and as a result the KKK was completely dismantled (ending the "first Klan" era) and did not resurface in any meaningful way until the beginning of the 20th century.[4]

Several of the act's provisions still exist today as codified statutes. The most important of these is 42 U.S.C. § 1983: Civil action for deprivation of rights. It is the most widely used civil rights enforcement statute, allowing people to sue in civil court over civil rights violations.