When President Obama addressed the American Society of News Editors convention last month, the real news was what didn’t happen. The watchdogs didn’t bark. No discouraging word from the gathering of 1,000 of the country’s top news people, facing a president whose administration has led a vigorous attack on journalism’s most indispensable asset — its sources.

Obama took office pledging tolerance and even support for whistleblowers, but instead is prosecuting them with a zeal that’s historically unprecedented. His Justice Department has conducted six prosecutions over leaks of classified information to reporters. Five involve the Espionage Act, a powerful law that had previously been used only four times since it was enacted in 1917 to prosecute spies.

Some spies. We’re no longer in the era of Aldrich Ames, Robert Hanssen or Kim Philby, infamous Cold War turncoats.

Instead, there’s Thomas Drake, a career official of the National Security Agency, who faced 35 years in prison for telling a Baltimore Sun reporter about what The New York Times called “a potential billion-dollar computer boondoggle.” At stake was bureaucratic embarrassment, not national security. (The case against Drake collapsed last summer.)

Then there’s Shamai Leibowitz, a translator for the FBI, who believed he had intercepted evidence of illegal influence-peddling by the Israeli embassy. When his boss wouldn’t act, he leaked transcripts to a blogger. He got 20 months.

Ex-CIA agent John Kiriakou was indicted in January for allegedly identifying a Guantanamo interrogator (who was not working undercover); Stephen Kim, a State Department analyst, allegedly told a reporter for Fox News — wait for it — that the U.S. was worried North Korea might respond to new U.N. sanctions by testing another A-bomb; and Jeffrey Sterling, who allegedly disclosed a botched CIA operation in Iran that was described in a 2006 book by a Times reporter.

And there’s the biggest case, the court martial of Bradley Manning, the Army private accused of engineering the mammoth dumps of U.S. military and diplomatic data that WikiLeaks, the online whistleblower network, turned over to leading newspapers in 2010 and 2011.

The administration seems undeterred by the scanty evidence that any of these defendants was out to hurt the country, a mainstay ingredient of espionage, and the Manning judge has even warned prosecutors they must show he believed he was “aiding the enemy” or she would toss the most serious charge against him.

The public is generally unaware of how essential nominally classified information is to coverage of diplomatic and strategic news. As Steven Aftergood, director of the Federation of American Scientists’ government secrecy project, put it: “The administration’s aggressive pursuit of leaks represents a challenge to the practice of national security reporting, which depends on the availability of unauthorized sources if it is to produce something more than ‘authorized’ news.”

What’s behind the administration’s fervor isn’t clear, but the news media have largely rolled over and yawned. A big reason is that prosecutors aren’t hassling reporters as they once did. Thanks to the post-9/11 explosion in government intercepts, electronic surveillance, and data capture of all imaginable kinds — the NSA is estimated to have intercepted 15-20 trillion communications in the past decade — the secrecy police have vast new ways to identify leakers.

So they no longer have to force journalists to expose confidential sources. As a national security representative told Lucy Dalglish, director of the Reporters Committee for Freedom of the Press, “We’re not going to subpoena reporters in the future. We don’t need to. We know who you’re talking to.”

It doesn’t appear that the current prosecutions required the help of journalists, which helps explain the ASNE’s equanimity when President Obama met the press last month.

But that silence constitutes an abdication of the media’s role as a voice in shaping public policy. After all, the ultimate purpose of reporter shield laws and the defiant tradition of protecting confidential sources isn’t to make writing stories easier for reporters, it’s to ensure that publicly significant information comes to light.

If the news media publish sensitive information, fully believing it ought to be made public, how can they stand by without protest when the government punishes the people who furnished it?

They can’t. The government may have found a way to suppress the flow of news without ruffling the feathers of reporters, but that doesn’t absolve the media of their duty to speak out for that flow.

The challenge now is for the media to rediscover their voice.

• Wasserman is Knight professor of journalism ethics at Washington and Lee University.