What role has geography played in the long arc of US history from the 1820s to the 1910s? (as detailed as possible)

Geography was key to US history during this period for two reasons: expansionism and hegemony. In other words, America had an incessant urge to spread its people and power to new regions and wanted maintain control of areas where it was already dominant.
The first key event of this period was the Monroe Doctrine (1823). In this doctrine, the United States declared that the New World was no longer open to European colonization. The United States also promised not to interfere in European wars.
Another important development during this period was the acquisition of vast lands from Mexico. These were conquered in 1848. This acquisition exacerbated tensions between North and South. The North wanted new lands to be free states, and the South sought to extend slavery into them.
After the end of the Civil War in 1865, America settled the west. It built railroads that spanned the continent. Also, Alaska was purchased in 1867. In 1893, Frederick Jackson Turner published his "frontier thesis" of American history.
In 1898, America defeated Spain and became an imperial power. By 1900, it controlled Hawaii, Cuba, the Philippines, and many other territories. America had become a world power.