The U.S. Department of Defense is trying to plug a major intelligence leak; the Internet. Over the last decade, the Internet has become, for the military, a major tool for increasing productivity, and boosting morale. But theres a dark side. With over two million uniformed (active duty and reserve) Internet users, information is getting out, that should not be getting out. This is largely due to the ease with which military users can forward messages and files, or get their PCs infected with viruses and worms. 

Early on, the Department of Defense began monitoring all email using .mil and .gov accounts. Initially, that sometimes slowed down delivery of email, and many Department of Defense users got free commercial accounts to get around that, and the filtering (which didnt stay a secret long.) The Department of Defense also set up two clones of the Internet, using its own extensive communications facilities. NIPRNET was unclassified, but contained only traffic from Department of Defense users. Same with SIPRNET, but on this one, everything was encrypted, and you could discuss highly classified material, and even transmit the stuff. 

The Department of Defense has also become a major customer for anti-virus and other network protection software, and hardware. Theres so much bad stuff prowling around the Internet these days, some of it controlled by foreign military and intelligence organizations, that the need to reduce the American vulnerability had become a high priority project. Part of the defense plan is to loudly and officially forbid users from doing many common Internet tasks. For example, you cannot use your personal email account while on a Department of Defense PC. No file sharing (P2P, Bit Torrent, Etc.), no unauthorized software or hardware on those machines either. And, definitely, absolutely, no forwarding of messages or files using a Department of Defense, unless authorized to do so. In other words, no playing around with your Department of Defense PC, unless you have permission to do so. Users are also getting more information on potential Internet dangers. Ignorance has proved to be a major liability in this area. So ignorance is being attacked with more education, more rules, and harsh punishments for those who break them. 

The prohibitions and rules vary somewhat from service to service, and even within a service. The Department of Defense doesnt expect to eliminate all vulnerability, but to reduce it as much as possible.