It was the most dramatic moment of the weeks-long trial in 2013 in which whistleblower Chelsea Manning was prosecuted for a historic leak of state secrets.

She was facing court martial for having electronically transferred a mind-boggling 250,000 US state department cables, almost half a million war-logs from Iraq and Afghanistan and a video of a US helicopter attack on civilians in Baghdad, to Julian Assange of WikiLeaks.

Manning, dressed in US military uniform and presenting as the male gender of her birth (she later transitioned as a woman), took the witness stand at Fort Meade military base in Maryland. Clutching 35 pages of personal statement, she began to read to a rapt courtroom.

She began by describing her deployment as an intelligence analyst in late 2009 to a military base outside Baghdad. She related pouring over vast amounts of secret data, and how, over time, she became disturbed by the US obsession with killing targets rather than the wider goals of peace and stability.

Manning went on to tell the court that in February 2010 she began talking through a secure online chat log to an individual with WikiLeaks going by the handle “Ox”. The reference will probably prove central to the prosecution case now being compiled against the WikiLeaks founder: the US government argues that the mysterious “Ox” was Assange, and that he actively conspired with Manning to steal US state secrets.

In the indictment released on Thursday, prosecutors accuse Assange of entering into a conspiracy with Manning through the webchats to “collaborate on the acquisition and dissemination of the classified records”.

Against that argument, WikiLeaks and a raft of prominent legal scholars argue that the relationship between him and Manning was purely that of a journalist and his source. A strong defense is certain to be mounted on Assange’s behalf under the first amendment of the US constitution that protects freedom of publication and dissemination of information to the public.

The lengths to which the US government is prepared to go to nail Assange for the 2010 leaks is now becoming clear. Not only have extradition proceedings been instigated against Assange, but Manning herself is back behind bars, having refused to testify before the US grand jury investigating WikiLeaks. She was recently subjected to a spell in solitary confinement at William G Truesdale adult detention center, in Alexandria, Virginia.

As the case thickens, the spotlight is likely to fall increasingly on those internet communications between Ox and his source. In her statement to the court martial, Manning said: “At first our conversations were general in nature, but over time as our conversation progressed, I assessed this individual to be an important part of” WikiLeaks.

In the webchats, Ox also went under the pseudonym “pressassociation”. Manning called him “Nathaniel Frank”, after the author of a book she had been reading.

“Over the next few months, I stayed in frequent contact with Nathaniel,” she said in her statement. “We conversed on nearly a daily basis and I felt that we were developing a friendship.”

But she noted acerbically that “in retrospect these dynamics were artificial and were valued more by myself than Nathaniel”.

Prosecutors seek to show that Assange’s conversations with Manning went beyond the activities of a journalist talking to a source and crossed the line into criminal conspiracy. They allege the WikiLeaks founder helped Manning crack a password that would allow the soldier to gain access to secret military databases using a false username.

The charges also allege that Assange took steps to hide the identity of Manning as the source of the leak and to “encourage Manning to provide information and records from departments and agencies of the United States”.

In a phrase that is likely to form a significant part of any eventual trial should Assange be successfully extradited, the prosecutors quote a web conversation from March 2010. The army private, under the handle “dawgnetwork”, said: “After this upload, that’s all I really have got left.”

Assange, as “pressassociation”, replied: “Curious eyes never run dry in my experience.”

The indictment also points to Assange discussing with Manning the value of the files on detainees in Guantanamo Bay that the soldier later downloaded to WikiLeaks. Manning referred to that conversation about the files, known as detainee assessment briefs (DABs), during her court martial statement.

“During my conversation with Nathaniel, I asked him if he thought the DABs were of any use to anyone. Nathaniel indicated, although he did not believe that they were of political significance, he did believe that they could be used to merge into the general historical account of what occurred at Guantanamo.”

Manning added: “After this discussion, I decided to download the data.”