From their website:
In the years following World War I, America was gripped by the fear that the Communist Revolution that had taken place in Russia would spread to the United States. As is often the case when fear outweighs rational debate, civil liberties paid the price. In November 1919 and January 1920, in what notoriously became known as the “Palmer Raids,” Attorney General Mitchell Palmer began rounding up and deporting so-called radicals. Thousands of people were arrested without warrants and without regard to constitutional protections against unlawful search and seizure. Those arrested were brutally treated and held in horrible conditions.
In the face of these egregious civil liberties abuses, a small group of people decided to take a stand, and thus was born the American Civil Liberties Union.Read more about their history
Main Page: American Civil Liberties Union