This comprehensive study treats the Iran–Russia–US relationship as a single, evolving triangle. The Lion, the Bear, and the Eagle narrates this history across five centuries. Drawing on multilingual archives, declassified intelligence, and interviews with senior officials, Arman Mahmoudian argues that Iran's realignments follow a consistent strategic logic, rooted in Iranian identity, that he terms the “Persian Measure.” By tracing this logic from the Safavid alliance with Russia in 1512 to today, the book offers a single lens through which to read half a millennium of shifting allegiances. Why does Iran align with Russia today, when forty-seven years ago it stood with the United States against the Soviet Union? The answer runs deeper than any single alliance. From the 1909 Siege of Tabriz to the 2026 Third Persian Gulf War, this book seeks to decode the logic that drives geopolitics' most consequential triangle.