Answer:
The fact that several nomadic empires, such as the Mongol Empire, have been short-lived is mainly based on the lack of defense structures built in a planned, strategic and orderly manner.
Due to their nomadic nature, these empires and civilizations did not tend to establish new cities or urban centers, with which their conception of territorial expansion was the mere possession of the territory. Therefore, in the event of enemy attacks, the territories were not sufficiently protected to repel these types of attacks. Thus, these empires could not sustain their power because they did not have the necessary building structures for it.
The continental divide runs along the length of the Rocky Mountains”
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Because it provided them with guns and rum.
The answer is 1918' or if you are going to the nearest decade that would be the 1920's.