Joyce v Director of Public Prosecutions AC 347

William Joyce was charged with treason for having made propaganda broadcast to the UK since Germany for the German Government. The accused was US citizen born in the US, it was contended that treason requires allegiance. As he had a UK Passport it his name at the time of broadcasting, was entitled to protection by the Crown and owed allegiance.

Treason can be committed within or without the realm. A state cannot ignored treason if committed outside its territory, on the contrary a proper regard for its own security requires that all those who commit the crime, should be amenable to its laws [Protective principle]