The Louisiana Purchase of 1803 was the single largest increase of territory in American history. The purchase took the form of a treaty between the United States and France which saw 828,800 square miles of land transferred to the U.S. in exchange for $11.25 million and a cancelation of debts equal to $3.75 million. The purchase is one of the key events in the administration of Thomas Jefferson and went a considerable way to forming the map of the modern United States. Despite this, the action was controversial at the time and many critics considered it unconstitutional.
Manifest Destiny was the belief that white Americans had a "God given" right to settle the western plains. The concept has been ever present throughout the history of the U.S., having first been invoked by Christopher Columbus on sailing to the New World, and the Pilgrims when they first alighted on Plymouth Rock. It was in the 1840s when the expansionist prophesy of manifest destiny began to be fulfilled, with legislation such as the Homestead Act, which allowed settlers to purchase plots of plains land at a low price on the condition that they live on and tend to the land for a minimum of five years. This led to numerous communities springing up on the Plains, although the West was still a long way from being "settled."
With the westward expansion, the new settlers began to discover vast quantities of natural resources beneath the extensive plains. This led to rushes toward certain areas of the continent as people sought to cash in on the as-yet untapped resources below ground. The gold rushes of 1849 are well-known, but similar -- if slightly smaller scale -- scenes occurred in the Minnesota-Lake Superior regions in the 1850s, as prospectors flocked to be the first to sink a mine and control the iron ore deposits that were found there. This led to an explosion in industrial activity, particularly in locations outside the traditionally larger cities, boosting the economy of the U.S. throughout the union.
Despite its new found wealth, territory and resources, America was slow off the mark compared to the other industrialized nations of the world to employ steel in the country's infrastructure. When Cornelius "Commodore" Vanderbilt began using steel rails for his trains in the 1870s, the lack of steel in the U.S. meant he had to import it from England. However, when steel was embraced it kick-started a second wave of industrialization. Quick to produce, steel railway tracks began to reach out in an irresistible advance across the nation, linking the previously isolated communities that sprang up following the Homestead Act of the 1840s. This transport network accelerated productivity in these communities and laid the groundwork for increasing industrialization in the first part of the 20th century.