In 2008, a foreign intelligence agency slipped a flash drive into a U.S. military computer, sneaking malicious code onto classified military networks. The code was loaded onto a military laptop in the Middle East and went undetected as it spread across classified and unclassified networks, waiting quietly to send sensitive information to foreign servers. The breach led the Department of Defense to issue a total ban on USB drives in late 2008, which has since been changed. One military official, speaking anonymously, explained the importance of keeping these networks secure: “This is how we order people to go to war””]
Source: https://gizmodo.com/americas-most-dangerous-computer-security-breach-was-ca-5621642