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A lawyer for the US government made the claim at Woolwich Crown Court, in London, on the opening day of Mr Assange's extradition hearing.
The court was told the Australian, 48, was guilty of "straightforward" criminality for hacking into and publishing US military databases.
He says it is politically motivated.
Mr Assange's lawyers say he could face up to 175 years in prison in the US if found guilty of one of the world's biggest data breaches, but James Lewis QC, for the US, described this as "hyperbole".