This article assesses the dynamic political and military relationships among Tibet, China, and India in the late 1950s and early 1960s. By examining the three governments' calculations and security interests, the article shows that the relationships among the three are best understood from a realist perspective. The focus in the article is on the Sino-Indian dispute over the territory known as “Assam Himalaya,” located on the far eastern end of the Sino-Indian border, between southeastern Tibet and northeast India. The article covers a relatively lengthy period, from 1913 to 1962, but in doing so it shows that territorial claims and the desire for secure borders were the key concern of all the countries involved—Tibet, China, India under British imperial rule, post-1947 India, and the United States.