The decade of the 1840s was the period in which the continental United States took on essentially its present boundaries, a process completed with the Gadsden Purchase a few years later. It was the golden age of Manifest Destiny. At the beginning of the decade, it was by no means a foregone conclusion that the United States would stretch "from sea to shining sea." By the end of the decade, the continent wide expansion of the country was established fact.
At the same time, however, the decade of the 1840s saw the slavery issue pushed to the forefront of American political life, and it was an issue intimately connected with the question of westward expansion. As vast new territories were brought into the union, slavery and anti slavery interests came into confrontation over whether the annexed lands would be settled as "free" or "slave" territories which, down the road, would necessarily imply their admission to the Union as either "free" or "slave" states, and thus alter the future power balance between the regions.
At the beginning of the 1840s, Americans were moving westward across the Great Plains at an accelerating rate, but it was not certain that this would lead to a political expansion of the United States. What is now the American Southwest, including California, belonged to Mexico. North of California, Spain and Russia had withdrawn from territorial contention, but competition remained between Britain and the United States. In this competition, Britain had the early advantage with its settlement at Ft. Vancouver (now Vancouver, WA, north of Portland), its trade links and, in the event of a conflict, the overwhelming power and reach of the Royal Navy.
Even if American settlement and culture prevailed, this did not necessarily imply the territorial expansion of U.S. political rule. After winning independence from Mexico, Texas had become an independent republic no "provisional" a<...