[dragonwriter]

> It seems that the federal government is trying to prosecute Assange for obtaining classified information from a source

No, they are prosecuting him for conspiring with Manning to gain access to classified material to which Manning did not have lawful access.

Conspiracy to commit a crime involves two or more people agreeing to a common purpose, which purpose involves a crime, and at least one of those people taking at least one overt act to advance toward acheiving the criminal purpose. (Note they the overt act can be by any member of the conspiracy, and all of them.can be charged.)