One hundred years ago this month, my great-great grandfather sat down to compose a letter which would finish a long and distinguished career — and destroy his reputation. Henry Petty-Fitzmaurice, 5th Marquess of Lansdowne, had held some of the most prominent posts in the British Empire and government: governor–general of Canada, viceroy of India, secretary of state for war, foreign secretary and Conservative leader of the House of Lords.