A top Pentagon official, William J. Lynn 3d, has confirmed a previously classified incident that he describes as “the most significant breach of U.S. military computers ever,” a 2008 episode in which a foreign intelligence agent used a flash drive to infect computers.

The infected flash drive was inserted into a U.S. military laptop at a base in the Middle East. The flash drive contained malicious computer code which uploaded itself onto a network run by the U.S. Central Command. That code spread undetected on both classified and unclassified systems, establishing what amounted to a digital beachhead, from which data could be transferred to servers under foreign control.

Mr. Lynn described the extraordinary difficulty of protecting military digital communications over a web of 15,000 networks and 7 million computing devices in dozens of countries against farflung adversaries who, with modest means and a reasonable degree of ingenuity, can inflict outsized damage.

Sources: The New York Times, Foreign Affairs