After three years characterised by delay, obfuscation and mistreatment, the trial of US army whistleblower Bradley Manning has finally commenced at a military facility in Maryland.

The material released by Manning has been of historic significance. It has revealed war crimes in Iraq and Afghanistan, the casual approach of US forces to the killing of unarmed civilians, the involvement of the US in military operations hitherto kept secret, the systematic use of the State Department to aggressively pursue the economic interests of the US without regard to cost, and the close involvement of non-Americans in US diplomatic and intelligence circles.

The critics of Manning and the media outlet that released the material, WikiLeaks, have vacillated between claiming the material revealed nothing of significance and that its release has been profoundly damaging and placed lives in danger. No evidence has ever been produced to justify the latter claim. US officials have admitted that the material has proven “embarrassing”.

The prosecution of Manning is just one of many prosecutions the administration of Barack Obama has undertaken against whistleblowers. But it is the most important one, because it heralded a broader assault on the media, particularly in the charging of Manning with “aiding the enemy”.

Manning’s supporters, Julian Assange and WikiLeaks supporters (including Daniel Ellsberg), have for more than two years warned that the prosecution of Manning, attempts by the US government to portray Assange as a co-conspirator with Manning, and the continuing investigation of WikiLeaks (an investigation publicly acknowledged by US officials, but absurdly denied by our Foreign Minister Bob Carr) reflected an agenda by the Obama administration to undermine investigative journalism. Those warnings about the threat posed by this prosecution to a free press were widely ignored, in some cases because critics wished to dispute WikiLeaks’ status as a media organisation or Manning’s status as a whistleblower.

In recent weeks, however, with revelations of the wiretapping and subpoenaing of journalists’ phone records from two high-profile media organisations, those warnings have proven prescient. The US government has engaged in an assault not merely on whistleblowers, but on journalists.

And what is why the trial of Bradley Manning matters, and will continue to matter.