In 1945, the United States dropped two nuclear bombs on Japan. It was devastating, historic and, ultimately, ended the Second World War. Some 70 years later, the frightening prospect of nuclear weapons falling into hands of terrorist organizations (think ISIS or the Taliban), who have proven their appetite for brutality again and again. On Thursday, President Barack Obama will host his fourth—and final—Nuclear Security Summit in Washington, where more than 50 heads of state will entertain that very notion, and how to ensure it never happens. Two key world figures are not attending: Russian President Vladimir Putin and Iranian President Hassan Rouhani. Iran’s absence appears more notable given the landmark nuclear deal with America and five other world powers. Experts say approaching such a terrifying possibility requires rethinking how we cope with the existence of nuclear arms. The Cold War mentality must make way for a far more fractured globe and the rise of ultra-fundamentalist Islam.