Treason of the Heart is an account of British people who took up foreign causes. Not mercenaries, then, but ideologues. Almost all were what today we would call radicals or activists, who thought they knew better than whichever bunch of backward or oppressed people it was that they had come to save. Usually they were applying to others what they saw as the benefits of their culture, and so obviously meritorious was their culture that they were prepared to be violent in imposing it. Some genuinely hated their own country, however, and saw themselves promoting abroad the values their own retrograde government was blocking.
The book deals with those like Thomas Paine who saw American independence as the surest means to hurt England; the many who hoped to spread the French revolution and then have Napoleon conquer England; historic characters like Lord Byron and Lawrence of Arabia who fought for the causes that brought them glory; finally those who took up Communism or Nazism. Treason of the Heart is nothing less than the tale of intellectuals deluded about the effect of what they are doing – and therefore with immediate reference to today's world.