At times, travel brings you face-to-face with some of the darkest periods of human history. A visit to Choeung Ek, the best-known memorial to the victims of the killing fields, is not particularly pleasant — its center is a Buddhist stupa with acrylic sides filled with more than 5,000 human skulls — but on a first visit to Cambodia it is somehow a necessary and unavoidable experience. The same is true of Yad Vashem, the official memorial to the Holocaust, which is located on the peaceful wooded slopes of Mount Herzl, on the western outskirts of Jerusalem. If you go to Israel, a visit seems mandatory.