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Answer: Mass migrations - Whenever a significant number of people leave one area and migrate to another, change occurs for both the land that they left as well as their destination
Imperial conquests - If an empire (or later a country) deliberately conquers territory outside its borders, significant changes tend to follow for both the attackers and the attacked.
Cross-cultural trade and exchange - Widespread contact among various areas of the world brings not only new goods but new ideas and customs to all areas involved.
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