One of the Pentagon's e-mail systems was hacked and it triggered the Pentagon to take off 1,500 e-mails accounts offline. However, the system does not contain classified information on military operation. Pentagon being one of the most advance military organisation in the world receives hundred of threats to its computer systems either from the young or adventurous hackers to the terrorists.
What is the best prevention method? Just ask US Defense Secretary and his answer is: "I don't use e-mail."
Source: BBC News - Cyber attack on Pentagon's e-mail