Complex sentence: In the 1780s, the newly independent United States, whose western border stretched to the Mississippi River, reached an agreement with Spain for navigation rights on the river and was content to let the "feeble" colonial power stay in control of the area.
Split into simpler sentences: In the 1780s, the western border of the newly independent United States stretched to the Mississippi River. The United States reached an agreement with Spain for navigation rights on the river and was content to let the "feeble" colonial power stay in control of the area.