Manifest destiny...imperialism...reform...acquisition. These are all terms that represent a period of great expansion and conquest that literally shaped the United States during the mid to late 1800's. Texas and Hawaii were gained, and so was virtually the western half of the US through the Louisiana Purchase and one of the most crucial wars in American history--the Mexican War. Not only did the United States gain much new territory and power during these years, but it also adopted a foreign policy via the Monroe Doctrine and Roosevelt Corollary that is still in use today.
The simplest way to describe and encompass the westward movement and conquest is through the public ideology "manifest destiny". The term was coined by John O'Sullivan in an article supporting the annexation of Texas (Internet). Its truest meaning holds that westward US expansion is not only inevitable, but also divinely approved. It was used throughout the 1800's to justify the annexation of Texas, Oregon Territory, and Alaska (Internet).
Those people moving across the plains also claimed they were fulfilling their divine mission to populate and settle North America. This term inspired those who wanted to start a new life in the west to make it a mission to be completed immediately.
Another important term that defined the time period is imperialism. This can be described as
"The policy, practice, or advocacy of extending the power and dominion of a nation especially by direct territorial acquisitions or by gaining indirect control over the political or economic life of other areas" ("Imperialism," def. 2). This dominated politics of the United States, and to an affect it still does today. The reason for the United States' belief in imperialism are many, and in the mid-1800's they were because they wanted to boost the economy, gain more power and strength,