National Archives and Records Administration (NARA)

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This explanation is quoted from the NARA web page:

By 1848 the United States had acquired official title to the contigous land stretching westward to the Pacific, south to the Rio Grande, and north to the 49th parallel. Americans had long since explored and settled in many of these areas, but legitimate possession created an impetus for development that began to crystallize as other timely occurrences brought a greater influx of people to the West. The religious persecution of the Mormons had led them to begin their migration westward by this time. The discovery of gold would soon draw thousands more across the country.