New York Times Co. V. United States (1971)

In 1971 the New York Times Co. published classified government documents that were leaked to the press. The newspaper had obtained a copy of documents known as “The Pentagon Papers." The Pentagon Papers, officially known as “History of U.S. Decision-Making Process on Viet Nam Policy,” were illegally copied and obtained by the New York Times and the Washington Post. Acting at the Government's request, the United States district court in New York temporarily ordered the New York Times not to publish the documents. The Government claimed that the publication of the papers would endanger the security of the United States. The New York Times appealed the order to the United States Supreme Court, arguing that prior restraint violated the 1st Amendment, and that is how New York Times Co. V. United States began.  While the Government had good reason to argue that these papers could endanger national security, the newspaper companies had a better argument which was supported by the 1st amendment as to why they should be allowed to publish the documents.  After a 6-3 decision between the justices, the Court ruled in favor of the New York Times and allowed them to publish the "Pentagon Papers." 

 This court case must have been one of the hardest Supreme Court cases to rule upon in history.  Although I agree with the government's argument that publishing classified information is endangering national security, I think the newspapers had a slightly better argument because they had the 1st amendment to back them up, which is arguably the firmest amendment in terms of trying to getting around it.
  This was a landmark case because it sort of set in stone for cases to come that the first amendment is the presiding law over issues as serious as national security.