Since the start of the Manhattan Project in 1942, the world has accumulated 1,875 tons of nuclear explosive materials that can be used for making powerful bombs. Terrorists could engineer a devastating attack merely by gaining access to a few pounds of these materials, because the designs of simple nuclear weapons are no longer secret. Anxious officials in the United States and other countries have long sought to prevent such an attack by safeguarding or destroying unneeded plutonium and highly-enriched uranium. But some of this effort has been mismanaged and certain foreign governments have resisted taking precautions that Washington has secretly urged. In this series, we examine the faltering global effort to control dangerous nuclear explosives and shine a light on the gravest risks. Earlier stories revealed problems with a U.S. and Russia plan to destroy excess Cold War-era plutonium, and controversy over Japan’s continuing construction of the world’s largest plutonium factory.