The sticking point here is both in the nature of how Bradley Manning is being held; and the suspected reasons why.
The latest UN ruling (PDF) states that 'solitary confinement is a harsh measure which may cause serious psychological and physiological adverse effects on individuals regardless of their specific conditions.' This amounts to torture under international law.
If that wasn't bad enough, then the motivation behind it seals the deal.
It is widely believed that Bradley Manning is being incarcerated in such conditions, without adequate trial nor hearing, to make him talk. He reportedly provided classified military documents to Julian Assange, which were subsequently published on the WikiLeaks website.
The documents and, in particular, video footage embarrassed the US regime, as they provided evidence of human rights abuses in the Middle East. Nothing could be done to stop them going public, but the supposed source was an American citizen. Bradley Manning was quickly arrested and hidden behind closed doors in a military prison.
The problem for US prosecutors is proving that he did anything. Also there is a much bigger goal here. Even if Manning is guilty (and he should be presumed innocent until he is eventually found to be otherwise in a court of law), then he is simply one whistle-blower.
Julian Assange is the real catch. US officials have made no secret of the fact that they want him silenced. Some senior members of the administration have even called for both him and Bradley Manning to be killed.
However, no extradition can be served on Assange (who is an Australian citizen) without proof of a crime committed in America. He maintains that he's merely a journalist, who believes in openness and accountability.
What would help the US case enormously is Bradley Manning confessing something which implicated Julian Assange. The fear is that this is precisely what the torturous incarceration of Manning is about.
This is a view that has been hinted at by the UN's Special Rapporteur on Torture. In an open letter (PDF) to the United States, dated December 30th 2010, Juan Mendez wrote, 'The prolonged period of isolated confinement which Mr Manning has allegedly been subjected to for seven months is believed to have been imposed in an effort to coerce him into 'cooperation' with the authorities, allegedly for the purpose of persuading him to implicate others.'
If it can be established that the USA is definitely torturing Bradley Manning, in order to use his 'evidence' to arrest Julian Assange, then this is a second strike against Article 1 of the Convention.