The native American Tipi was originally a small tent used during the hunting season.
The indigenous people of Plains in the Great Plains and Canadian Prairies of North America usually made their tent out of animal skins. The tent was cone-shaped and made up of skins of animals which was hanged on the wooden poles. The Tipi was used by the people to receive warmth and comfort in winter, coolness in summers and it was durable too. It made the life of the nomadic people easier as it was an easy task to make a tipi for the purpose of getting shelter.
A major reason was that he wanted them gone from the land and they didn't want to move. He believed that the US had to go westwards while the Natives didn't want to go away from their land. This is after he signed the Indian Removal Act. Cherokee were one of the tribes that didn't want to relocate, and what happened later is history.
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I believe it was the defense the Chinese were implementing, from fear of the Moguls invading. So the Great was in place being built and the Government closed off trade and travel so no more boats were built.