Manning had bragged online that he downloaded 260,000 classified U.S. cables and transmitted them to Wikileaks.org.
Assange on Monday compared the impact of the released material to the opening of East Germany's secret police files. "This is the equivalent of opening the Stasi archives," he said.
He also said his group had many more documents on other subjects, including files on countries from across the globe.
"We have built up an enormous backlog of whistleblower disclosures," he said.
Assange said he believed more whistle-blowing material will flood in after the publicity about the Afghan files.
"It is our experience that courage is contagious," he said.