The Espionage Act was crafted in 1917 — because President Woodrow Wilson wanted a war and, faced with the troublesome First Amendment, wished to criminalize speech critical of his war. In the run-up to World War One, there were many ordinary citizens — educators, journalists, publishers, civil rights leaders, union activists — who were speaking out against US involvement in the war. The Espionage Act was used to round these citizens by the thousands for the newly minted ‘crime’ of their exercising their First Amendment Rights.
And, more specifically to applying it to Julian Assange as the publisher of the cables on wikileaks:
If you prosecute journalists — and Assange, let us remember, is the New York Times in the parallel case of the Pentagon Papers, not Daniel Ellsberg; he is the publisher, not the one who revealed the classified information — then any outlet, any citizen, who discusses or addresses ‘classified’ information can be arrested on ‘national security’ grounds. If Assange can be prosecuted under the Espionage Act, then so can the New York Times; and the producers of Parker Spitzer, who discussed the WikiLeaks material two nights ago; and the people who posted a mirror WikiLeaks site on my Facebook ‘fan’ page; and Fox News producers, who addressed the leak and summarized the content of the classified information; and every one of you who may have downloaded information about it; and so on.