The case has nothing to do with Russia’s election interference in 2016, when Mr. Assange’s organization published Democratic emails stolen by Russia as part of its covert efforts to help elect President Trump. Instead, it focuses on Mr. Assange’s role in the leak of hundreds of thousands of State Department cables and military files by the former Army intelligence analyst Chelsea Manning.

Justice Department officials did not explain why they decided to charge Mr. Assange under the Espionage Act — a step also debated within the Obama administration but ultimately not taken.