William, the duke of Normandy: 1048 to 1060
You refused to attend that peace conference. King Henry did NOT agree never to attack you within your realm again (which he was willing enough to promise, right after you had beaten him). The war continues unabated: Henry gets his courage and sense of outrage back again, and he and Anjou invade Normandy tenaciously. Of course, you win in the end, because both Anjou and the king die in 1060: but the time and resources it has cost you set your timetable back several years. When king Edward dies in 1066, you are in no position to invade England, because you still are involved in getting control of Maine and Brittany. Amazing, isn't it, how a little decision, like refusing to attend a peace conference, can cost you so much later on?
