Wikileaks obtained hundreds of thousands of secret American military and diplomatic documents from a U.S. soldier (PFC Bradley Manning) who worked in intelligence. As such, Manning had a security clearance and access to SIPRNet (Secret Internet Protocol Router Network). This was a private Department of Defense network established in 1991, using Internet technology and able to handle classified (secret) documents. But Manning got access to a computer with a writable CD drive, and was able to copy all those classified documents to a CD (marked as containing Lady Gaga tracks) and walk out of his workplace with it. The big error here was having PCs available with writable media (USB ports, diskettes, printers or writable CD drives). You need some PCs with these devices, but they should be few, and carefully monitored. Normally, you would not need to copy anything off SIPRNet. Most of the time, if you want to share something, it's with someone else on SIPRNet, so you can just email it to them, or tell them what it is so they can call it up themselves. A network like SIPRNet usually (in many corporations, and some government agencies) has software that monitors who accesses, and copies, documents, and reports any action that meets certain standards (of possibly being harmful). SIPRNet did not have these controls in place.
Diplomatic messages, at least some of the lower level classified stuff, was put on SIPRNet by a presidential directive that sought to get other departments sharing relevant data with military intelligence. This was to avoid the kind of bad communications that made possible the September 11, 2001 attacks. Before then, even though some American government agencies had prior information on the attackers, no one made the connection. Unless all this information is collected together to make it obvious what is going on, the attackers will go undetected until it is too late. As a result of the Manning leak, the State Department withdrew access to its material by SIPRNet.
In the last nine years, SIPRNet became extremely active, and what controls there were on the network were strained to the point that you could do just about anything. This sharing of information was very helpful in fighting Islamic terrorists. Yet, with 2.5 million troops and civilians having access to SIPRNet, there were very few leaks. All it takes is one person, though. For three years, Manning was a one man SIPRNet crime wave. He is now in jail, facing life in prison.
The leaked documents were meant (according to the Wikileaks leader) to embarrass the United States and expose American hypocrisy and underhanded operations, but the result was quite the opposite. The U.S. was shown trying to do what it said, publically, that it was trying to do. But many other nations were shown to be quite different in their private conversations, than in their public ones. Some of these leaders now claim that they were misquoted, or that Wikileaks documents were a fabrication. It was initially believed that the released documents would make foreign officials more reluctant about speaking frankly with American officials. Didn't happen. Those conversations take place mainly because everyone wants something from the United States, and unless you establish a relationship with American diplomats or officials, nothing will happen. Moreover, many foreign officials found the revelations useful, as the leaks got out into the open things (like Arab relationships with Iran and Israel) that could not be discussed openly at home. For the most part, Wikileaks confirmed what was already known, something the Wikileaks crew assumed could not be true.