hahahaha a super interesting question. In ancient China, espacial in literary, people used to use "Landscape"or "mountain and sea/river江山" to represent the territory of the kingdom. Power to control the whole territory is an abstract concept, but "mountain and river江山" is much more visual and thus always be used in analogy. Back to your example, 定江山 straightforward means "earn the power to control the whole kingdom". Who has the power? The emperor, the winner of competition. In modern world, this expression could be used in very casual conversation, and the "territory江山" even could be used to represent a firm, a house, a lunch , or anything be treated as an achievement. For instance, "来猜拳决定这个鸡腿谁来吃！一锤定江山！". Hope this answers your question! Yihang