In the 1930s and 1940s, American atomic physicists developed an identity akin to those ethnic identities developed by Chicanos and African Americans in the 1960s. Tremendous successes in high-energy physics put these American physicists at the pinnacle of science worldwide. Luis W. Alvarez was one of the central figures in this rise, was central to the development of “Big Science,” and won the Nobel Prize in 1968. However, historians have largely ignored him. Through Alvarez we see that American atomic physicists before the 1930s lacked an identity. Alvarez witnessed the growth of his field and was an early advocate for an identity for American atomic physicists. Using identity politics as a theme, we find five stories centered on Alvarez that illustrate this emerging self-image. Alvarez’s autobiography demonstrates his interest in preserving the history of physics and establishing his place in it. A textbook draft that Alvarez abandoned in 1952 further illustrates his early interest in the history of physics then absent in physics textbooks and an early interest in mythology and heroes. Alvarez’s work outside of physics helps define the boundaries of this newly self-identifying group as he conquered fields like forensics and pyramidology, as well as famously proposing the theory that an asteroid killed the dinosaurs. A collection of letters from cranks helps us demarcate science from non-science and thus define the boundaries of science. Finally, Alvarez’s identity as a physicist is contrasted with another category of identity, his ethnic identity. Alvarez was a white man with a Hispanic name, which provides us with the rare case of a white man discussing his whiteness with would-be biographers who wanted to frame him as a “Chicano physicist.” Altogether, Alvarez would, much more than any physicist in his generation, promoted and exemplified an identity as an American atomic physicist while rejecting other identities. / text