The Open Door Policy in China was proposed by the U.S. government in 1899. Although adopted to stop the foreign partition of China, it was condemned as economic imperialism during the Cold-War period. With the People's Republic of China (PRC) embracing market reforms, encouraging foreign investment, and promoting capitalist growth in the 21 st Century, this book examines and re-evaluates the former economically-based critiques of the Open Door Policy, from its inception in 1899 to its collapse in the 1920s. It offers new evidence suggesting the hitherto underestimated role of the Open Door Policy in protecting China's territorial integrity from Russian and Japanese encroachment. Using primary documents located in the Peking government's Foreign Ministry archives in Taipei, Taiwan, the Ministry of Foreign Affairs archive in Tokyo, Japan, and the Trotsky archives at Harvard University, United States, it sheds light on how the destruction of the Open Door Policy during the 1920s cleared the way for a resurgence of Russian and Japanese expansionism in China, ushering in decades of foreign invasion, civil war, and revolution, until the 1949 establishment of the PRC once again shored up China's threatened territorial integrity.