Book excerpt: On January 27, 2017, President Donald Trump directed Secretary of Defense James Mattis to initiate a new Nuclear Posture Review (NPR). The President made clear that his first priority is to protect the United States, allies, and partners. He also emphasized both the long-term goal of eliminating nuclear weapons and the requirement that the United States have modern, flexible, and resilient nuclear capabilities that are safe and secure until such a time as nuclear weapons can prudently be eliminated from the world.The United States remains committed to its efforts in support of the ultimate global elimination of nuclear, biological, and chemical weapons. It has reduced the nuclear stockpile by over 85 percent since the height of the Cold War and deployed no new nuclear capabilities for over two decades. Nevertheless, global threat conditions have worsened markedly since the most recent 2010 NPR, including increasingly explicit nuclear threats from potential adversaries. The United States now faces a more diverse and advanced nuclear-threat environment than ever before, with considerable dynamism in potential adversaries' development and deployment programs for nuclear weapons and delivery systems.