Chapter 4 discusses the transition from Ly to Tran dynasties in the early thirteenth century as a result of the rise of the Jiaozhi Sea. It marks a fundamental shift in political power from the upper-mid Red River valley to the coast. The Ly Dai Viet was a mid-river principality that ruled no more than half of the delta. By the middle of the twelfth century, Ly Dai Viet was looking increasingly seaward. Changing external factors played a major role in accelerating this transition. The main drive was Southern Song dynasty’s need for Yunnan horse that brought huge cash to the Guangxi border facing the Gulf of Tongking. Jiaozhi Sea networks flourished. Dai Viet’s main port shifted from the central Vietnam to Van Don, where merchants from Champa traded aromatics with the silk merchants from Sichuan. It was against this background that the Tran family from Fujian rose and replaced the Ly dynasty. The rise of the Jiaozhi Sea and resurgence of the Tongking Gulf accelerated the integration of the political forces of the upper, mid, and lower Red River delta and placed Tran-era Dai Viet in a strategic position to respond to greater macro-regional changes, not only on the agricultural and population but also the expansion of Islamic trade networks.