Final answer:
Before the Louisiana Purchase, the eastern boundary of the United States was the Atlantic Ocean, and the western boundary was the Mississippi River. The purchase doubled the nation's size, leading to westward expansion that would deeply impact Native American tribes and set the stage for further territorial disputes and expansion.
Step-by-step explanation:
Before the Louisiana Purchase in 1803, the eastern boundary of the United States was the Atlantic Ocean. The western boundary was the Mississippi River, which separated U.S. territory from the Spanish-controlled region to the west. It was this purchase under President Thomas Jefferson that dramatically expanded the country's boundaries and sparked a significant westward expansion. At the time, the eastern portion of the continent was more heavily settled, while the west held promises of cheap land, minerals, and other financial opportunities. This western expansion was not without consequences, as it led to the displacement of Native American tribes, pushing them further west onto reservations.
Significant figures like Meriwether Lewis, who was appointed by Jefferson to explore these new lands, and events such as the invention of the cotton gin in 1793, which fueled the expansion of plantation slavery, all played critical roles in the transformation of the United States during this time. The acquisition of Florida through the Adams-Onís Treaty and the subsequent attempts by settlers to expand into Texas further evidenced the strong drive for American expansionism prior to the concept of Manifest Destiny later in the 19th century.