What do national security and defense mean in the ecologically destabilizing age of climate change? In California, the US military has begun to redefine these concepts by taking on a largely unrecognized yet crucial role in renewable-energy innovation and in preserving cultural and natural treasures. Environmental stewardship is law on installations throughout the United States, but a few bases in Southern California have taken a more comprehensive approach—one in which energy security and protection of threatened and endangered species are embedded in the practice of national defense. Unlikely Ally takes us through these bases to examine what twenty-first-century sustainable-energy infrastructure looks like; whether combat readiness and species protection can successfully coexist; how cutting-edge technology and water-conservation practices could transform life in a resource-constrained world; and how the Department of Defense’s scientific research into the metabolic secrets of the endangered desert tortoise could speed human travel to Mars. With investigative journalist Marilyn Berlin Snell as our guide, we explore a martial culture in California informed by science, strategic imperative, state and federal law, and visionary leadership.