The men were first arrested in March 2023 on suspicion of offences under the Official Secrets Act - and if the case were to go before a jury, the prosecution would have to prove that the defendants had carried out activity "prejudicial to the safety or interests of the State".
Secondly, a jury would also have to be sure that the UK government had considered between 2021 and 2023 - when the alleged offending occurred - that China was an "enemy".
Prosecutors concluded that they would need to show a jury that China was an "enemy" with the help of expert factual evidence from Matt Collins, the Deputy National Security Adviser (DNSA).
As DNSA, Mr Collins is responsible for coming up with an assessment of threats to the UK's national security.
He began drafting, with advice from his own lawyers and other officials, a statement which had to be solely focused on the then Conservative government's official and publicly disclosable conclusions about the threat, if any, that China posed.
This evidence is separate from any information generated by the intelligence services that remains secret.
The eventual statement went into detail about the activities of Chinese intelligence agencies and how they seek to obtain information about the UK's political workings - but the word "enemy" was removed by the time a final version was completed in December 2023 and shared with Downing Street.
Mr Collins, in his own letter to MPs, said he told police investigating the case he could not call China an "enemy" as this "did not reflect government policy".
In July 2024, a Court of Appeal ruling on the legal definition of enemy, in a separate case concerning Russian interference in the UK, set off alarm bells in the CPS.
It underlined the need to provide a jury with a factual account of why a state could be considered an enemy under the Official Secrets Act - and while the government had provided a clear conclusion about Russia, it had not done so for China.
In his letter to MPs, Parkinson said that ruling meant the CPS and police had to go back to the DNSA to ask him for more evidence about China.
That evidence was essential because prosecutors knew that the defendant's barristers would question whether there was no evidence at all that the UK had regarded China overall as a threat between 2021 and 2023.
