When the term first came into widespread use in medieval England, a "writ of habeas corpus" was simply a subpoena. A king or local official could impose a "writ of habeas corpus" to force someone to appear and testify. But over time, it took on a more libertarian meaning. By the time England's Habeas Corpus Act came into effect in 1679, it was understood as a civil right protecting the populace from being arrested without charge.
Violation of habeas corpus is not in and of itself the most severe civil liberties violation imaginable, but it can be used to disguise any number of more severe civil liberties violations. When Adolf Hitler signed the Nacht und Nebel ("Night and Fog") Decree of December 1941, essentially abolishing habeas corpus in cases where anti-government sentiment was suspected, it helped disguise some of the earlier stages of the Holocaust. And the Soviet Union's frequent disregard for habeas corpus, particularly during and following the Stalin era, allowed for the imprisonment of dissidents on a frightening scale.
Today, the most frequently discussed international habeas corpus controversy is the Bush administration's policy of imprisoning hundreds of suspected Afghan and Iraqi terrorist associates in Guantanamo Bay and other offshore U.S. prisons without trial or meaningful judicial review. Both President-Elect Barack Obama and the Democratic Congress have promised to reestablish habeas corpus following Obama's January 2009 inauguration, but whether they will follow through on this promise remains to be seen.