It was Manning who sent him the files, it is not Assange’s crime for accepting them- how was he to know what Manning was sending him?
Why is Manning not in front of a firing squad for getting operatives killed?
The government is alleging that they have chat logs that show that Assange was colluding with Manning before and while Manning was acquiring the files, and advising him on how to get past some of the security measures.
There was some testimony alleging Assange paid Manning for the stolen info.