The Vietnamese trace the origins of their culture and nation to the fertile plains of the Red River Delta in northern Vietnam. After centuries of developing civilization and economy based on the cultivation of irrigated rice, in the tenth century, the Vietnamese began expanding southward in search of new rice lands. Until the mid-nineteenth century, the Vietnamese gradually moved down the narrow coastal plain of the Indochina Peninsula, ultimately extending their reach into the broad Mekong River Delta. Vietnamese history is the story of the struggle to develop a sense of nationhood throughout this narrow, 1,500-kilometer stretch of land and to maintain it against internal and external pressures. China was the chief source of Vietnam’s foreign ideas and the earliest threat to its national sovereignty. As a result of a millennium of Chinese control beginning in about 111 BC, the Vietnamese assimilated Chinese influence in the areas of administration, law, education, literature, language, and culture.