The 20th century was marked by immense human progress but was also punctuated by episodes of grave inhumanity. At the conclusion of its third and final meeting, held in Phnom Penh, the CSCAP Study Group on the Responsibility to Protect (RtoP) visited the Tuol Sleng genocide museum, site of one of the century’s worst crimes. During the Khmer Rouge’s reign of terror in Cambodia, up to 17,000 people were tortured and killed at Tuol Sleng. Despite past promises, the international community has often failed to prevent the commission of crimes like this or take timely and decisive action to protect the victims. Recognizing this legacy, the largest ever meeting of heads of state and government endorsed the Responsibility to Protect (RtoP) concept in 2005.