People’s History of the United States has this to say about Ellsberg’s leak of the Pentagon Papers:
Ellsberg and Russo spent night after night, after hours, at a friend’s advertising agency, duplicating the 7,000-page document. Then Ellsberg gave copies to various Congressmen and to the New York Times. In June 1971 the Times began printing selections from what came to be known as the Pentagon Papers. It created a national sensation.
The Nixon administration tried to get the Supreme Court to stop further publication, but the Court said this was “prior restraint” of the freedom of the press and thus unconstitutional. The government then indicted Ellsberg and Russo for violating the Espionage Act by releasing classified documents to unauthorized people; they faced long terms in prison if convicted. The judge, however, called off the trial during the jury deliberations, because the Watergate events unfolding at the time revealed unfair practices by the prosecution.
Ellsberg, by his bold act, had broken with the usual tactic of dissidents inside the government who bided their time and kept their opinions to themselves, hoping for small changes in policy.