Yes, it is true that the new frontier extended from the Mississippi River westward to the Pacific Ocean.
What was the westward expansion of the Pacific Ocean?
The United States' 19th-century westward expansion encompassed not just its advance across North America but also its continuous endeavor to fortify its position in and across the Pacific. The majority of this marine growth was spurred by business, which had significant effects on US foreign policy.
With the Louisiana Purchase of 1803, the United States' territory was significantly increased, and its western border was stretched to the Mississippi River. Following that, the United States continued to expand westward as a result of a series of actions that included the Lewis and Clark expedition and the acquisition of additional land, including the Oregon land.