Final answer:
The U.S. policy of trans-Appalachian westward expansion was encouraged by the passage of the Northwest Ordinance, while it had significant impacts on both relations with American Indians and international relations from 1780 to 1800.
Step-by-step explanation:
a. One way U.S. policy encouraged trans-Appalachian westward expansion from 1780 to 1800 was through the passing of the Northwest Ordinance in 1787. This policy established a process for admitting new states to the Union, which encouraged settlers to move west as they knew there would be opportunities for statehood and self-governance.
b. Trans-Appalachian westward expansion had a significant impact on relations between the United States and American Indians during this period. The expansion caused increased conflicts and tensions between settlers and Native American tribes as settlers encroached on their lands and disrupted their way of life. This ultimately led to forced removals, such as the Trail of Tears, and the loss of Native American territories.
c. Trans-Appalachian westward expansion also had an impact on international relations from 1780 to 1800. The United States' expansion into territories like the Louisiana Purchase and along the west coast brought them into contact with other global powers, such as France and Spain. This expansion and control over key trading ports and routes strengthened the United States' position on the global stage and increased its influence in international affairs.