In Political Sciences, forecasting is taken as a crucial method in studying international relations. This is a scientifically-based research process that seeks to understand and embrace the movement trend, the fundamental development trend of the future and to recommend a number of different responses. This article examines the methodology of forecasting based on the materialist dialectical methodology, predicting the movement and transformation of the world on the principle of inheritance. Inheritance is approached in three aspects: first, how the old world formed and developed; second, the appearance and impact of new elements in the new world; third, degree of reception of the new things and renouncement between the old world and the new world. At the same time, the authors use this method to forecast the trend of Southeast Asia in the coming decades of the 21st century. We anticipate that in the twentieth century, Southeast Asia was a colonial region of Western colonialism and also a hotbed of the Cold War. The inception of the Association of Southeast Asian Nations (ASEAN) in the last half century has contributed significantly to the change and development of Southeast Asia although there are ups and downs within which the aim of ASEAN has been consistent to peace, stability, cooperation and development. Therefore, peace, cooperation and development will continue to be the mainstream of Southeast Asian relations, but the region continues to be significantly geopolitical for great and major powers to compete for influence.
