In a statement posted on the WikiLeaks website, he said: "When communicating with the press is 'aiding the enemy' it is the 'general knowledge among the people' itself which has become criminal.
"Just as Bradley Manning is condemned, so too is that spirit of liberty in which America was founded.
"In the end it is not Bradley Manning who is on trial. His trial ended long ago. The defendant now, and for the next 12 weeks, is the United States.
"A runaway military, whose misdeeds have been laid bare, and a secretive government at war with the public. They sit in the docks. We are called to serve as jurists. We must not turn away."
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Former Chancellor Lord Geoffrey Howe was Margaret Thatcher's longest-serving Cabinet minister, also taking on the role of Foreign Secretary.
Lord Geoffrey Howe is known by many for his resignation speech which was seen as the catalyst which destroyed Margaret Thatcher.