A British man has been arrested in England and charged by the United States and Britain with infiltrating US government computer systems.
The systems included those run by the military, to steal confidential data and disrupt operations.
US prosecutors said the alleged hacker, Lauri Love, infiltrated thousands of computer systems including those of the Pentagon's Missile Defense Agency.
He also infiltrated the US Army Corps of Engineers, the US space agency NASA and the US Environmental Protection Agency.
Mr Love, 28, and three unnamed co-conspirators, believed to live in Australia and Sweden.
They intended to "disrupt the operations and infrastructure of the United States government," according to a US indictment unsealed today.
US Attorney Paul Fishman in New Jersey said: "Such conduct endangers the security of our country and is an affront to those who serve,"
Mr Love was charged in Britain with violating the Computer Misuse Act, and charged in the US with accessing a US government computer without permission and conspiracy, authorities said.
Mr Fishman said the hacking took place from October 2012 until this month.
He said it compromised personal data of US military personnel, and information on defence budgets, contract bidding, and the demolition and disposal of military facilities,and caused millions of dollars of losses.
Mr Love lives in the Suffolk village of Stradishall, and was arrested at his residence on 25 October by authorities including the cybercrime unit of Britain's National Crime Agency (NCA),authorities said.
He has been released on bail until February 2014, an NCA spokeswoman said.
The arrest comes as authorities worldwide coordinate efforts to combat cyber crime.
On 10 October, US Defense Secretary Chuck Hagel issued a memorandum emphasising a need to safeguard even unclassified technical data against cyber intrusions to help protect US military superiority.
US prosecutors said the scheme involved the installation by Mr Love and his co-conspirators of malware in the hacked systems, creating "shells" and "back doors" that allowed them to return later to steal data.
According to the US indictment, Love, who was also known as "nsh" and "route" and "peace," at times used internet chat rooms to discuss the intrusions and efforts to conceal them.
Elsewhere, British Prime Minister David Cameron threatened on Monday to act to stop newspapers publishing what he called damaging leaks from former US intelligence operative Edward Snowden.
Britain's Guardian newspaper had printed damaging material after initially agreeing to destroy other sensitive data, he said.
While making clear his patience was running out, Mr Cameron told politicians his preference was not to get heavy-handed with newspapers that published such information and that he hoped they would change their behaviour instead.