One of the facts selected for heavy emphasis in the recent transatlantic air plot was that the alleged plotters were one and all British subjects. Whether the conflict between Britain and al-Qaeda is a war or not is an interesting question, in this context. Of course, siding with the enemy of one's country in war is one of the classic examples of treason, as specified in a longstanding English statute.
In previous conflicts, the British state has had to cope with a similar situation to today, in having a a large pool of potential enemy sympathizers. Perhaps one parallel is the treatment of Oswald Mosley and his fascists in 1939. But perhaps one must look elsewhere for a better parallel. We need an internationally recognized regime, confronted in war by an extra-territorial organization, that is liable to recruit combatants from citizen-sympathizers, who may receive support from an external quasi-military formation: This parallel involves "the trials of 135,000 French people, the internment of 70,000 `enemies of the state', the complicity of the French police and the Milice in suppressing resistance" (quoted from here). It's not really a pretty picture, and is another of the many reasons for ordinary people, who will suffer most on a daily basis, not to enter into war eagerly.