One of the most important was climate changes. As climate changed, nomads had to find new places to migrate to which led to people going to areas that were uninhabited before. Another was the development of agriculture which more or less ended the nomadic way of life when people started settling.
In U.S there was a lot more land available at the time and many people were looking for a fresh start. Many people were looking to expand trades and business. In the city many places offered unskilled workers opportunities for a steady job.
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Answer:
The U.S. suffered badly but not as bad as the soviet union, China, poland, India, etc. The U.S lost . 1,027,000 to 1,700,000 lives, the soviet union lost 20,000,00 to 27,000,000 lives, etc.
Explanation:
