Yes, it is true that the new frontier extended from the Mississippi River westward to the Pacific Ocean.
What is the justification?
The idea of the "new frontier" throughout the 19th century described the westward expansion of US territory. Numerous factors, including the need for resources, land, and economic possibilities, propelled this expansion.
The United States' territory was greatly expanded and its western border was extended to the Mississippi River with the Louisiana Purchase of 1803. The United States then proceeded to grow westward through a sequence of events that included the Lewis and Clark expedition and the purchase of new territory, such as the Oregon Territory.