Why is the "westward movement" in the United States an Indian and a history of blood and tears?

The westward movement and territorial expansion are intertwined. In the westward movement, the western region was developed, which greatly promoted the economic development of the United States. However, with the westward movement, a large number of Indians were slaughtered, and the survivors were forcibly driven to a more desolate "reservation". Their forced migration road is also known as the "tears road" of Indians.