Final answer:
The Mississippi River served as the border and allowed the British military to keep its posts in the 1783 Treaty of Paris.
Step-by-step explanation:
The Mississippi River played a significant role in the 1783 Treaty of Paris. One of its roles was that it served as the official border for the new United States, marking the western boundary. Another role was that the British military was allowed to keep its posts along the river, maintaining their presence in the newly formed United States.
Learn more about 1783 Treaty of Paris