One of many Americans hired as advisors by the Chinese Nationalist government in the decade-and-a-half before Pearl Harbor, the famous cryptographer Herbert O. Yardley made crucial but long underestimated contributions to China's war effort. The Nationalist Government benefitted more in communications intelligence from recruiting Yardley than from other intelligence partnerships. Yardley's codebreaking work in China also offers a window into the transformation of Sino-American relations and the US role in Asia during the 1940s. Intelligence cooperation and covert operations became key tools of US statecraft in Asia and elsewhere around the globe during the Cold War. But before Pearl Harbor, Sino-US military and intelligence cooperation relied on partnerships between individual, non-state American actors, such as Yardley, and the Chinese government. This chapter's exploration of Yardley's work with the Juntong reveals how the ROC government's security needs and engagement with non-state actors influenced the origins and development of the US-dominated Cold War order in the Western Pacific.