Mike,
My understanding of the law of armed conflict is tiny, but caveat aside now.
There are several laws regarding armed conflict which appear to have universal jurisdiction, hence the arrest and conviction of an Afghan warlord, an asylum claimant in the UK for torture IIRC and recently a Nepalese Army officer for torture - whilst visiting on leave from a UN mission.
Do such laws have international jurisdiction themselves, or is it the result of incorporation into national law? I think it is the later for the UK.
Several activists have called for the UK authorities to take legal action against suspected sinners, most notably Peter Tatchell who called for Robert Mugabe to be arrested, indeed tried himself to make a "citizens arrest" and was cast aside by Mugabe's own bodyguards.
A few years ago a private application for a summons or a warrant was made to enable the arrest of an Israeli military officer, due to make a PR visit to the UK and the police declined to make an arrest when he landed @ Heathrow. The Director of Public Prosecutions (DPP, our national prosecutor for England & Wales), then stepped in, using a hitherto unknown legal power, to take over the case and have the summons / warrant dropped.
Whatever the desires of some activists and lawyers I cannot see the UK authorities asserting a universal jurisdiction or using a national criminal law to launch a prosecution. Yes citing the 'national interest' and 'national security' come to mind, but as with the USA it is remarkable how few, if any prosecutions have been made over national criminal offences in the financial sector. Why does anyone suppose the UK (English & Welsh) prosecution authorities will suddenly become paragons of virtue and brave too?
Now a civil action here is quite different and that is where the alliance of activists and human rights lawyers are "making hay", nibbling away at the government's arguments. Hence the attempt to enable civil court procedures to have information i.e intelligence material heard by the judge only, without challenge or disclosure to the plaintiff.
Bookmarks