The U.S. government provided guarantees to the British High Court of Justice that Julian Assange will receive a fair trial if he is taken from the Belmarsh maximum security prison outside of London, and extradited to the United States.
At the end of March, the High Court ruled that Assange had a strong legal basis to appeal his extradition in three specific aspects. The U.S. government, the verdict stated, must ensure that his freedom of speech under the First Amendment of its own Constitution will be protected; also, that the Australian publisher will receive a fair trial with full guarantees, and that he will not get the death penalty.
h/t ObjectiveObserver420