The discovery that the People's Republic of China is expanding its nuclear arsenal, Russia's "suspension" of the New START arms control treaty and aggression in Ukraine, and North Korea's evolving nuclear capabilities have led some to argue that the United States needs to expand its own nuclear forces to deter these potential adversaries. This Perspective articulates a framework for determining "how much is enough" for U.S. nuclear forces in the emerging strategic environment. The author argues that adversary perceptions, rather than quantitative comparisons, should be the primary criteria for sizing U.S. nuclear forces.