In the 1800's, Jefferson was elected as president of the United States. With this election, it caused a peaceful transition of power from the Federalists to the Democratic-Republicans. With a new president in office, the want to expand into western territory began to surface.
On April 30th, 1803, Jefferson purchased the Louisiana Territory from France. In 1804 Jefferson sent Lewis and Clark to explore the new territory. this fulfilled temporarily the want for westward expansion beyond the Mississippi River. This new land caused disputes over which new states would be slave or free.
The conflict over what state would be what would be solved through the Missouri Compromise. Missouri would be admitted as a slave state and Maine would be admitted as a free state. All states north of the 36 30 line would be free. This was disliked by the southern states, which lead Monroe to be re-elected by one vote
The Mexican-American War
The Monroe Doctrine was drafted by John Quincy Adams, stating that the Americas were no longer available for colonization by European powers. While this wouldn't settle which state was what, this would solidify them as an independent country, ready to become one of the leading industrialized country's
During the Industrial Revolution, the railroad would become one of the most important and significant developments toward a national economy. With this important development, Americans started to demand a transcontinental railroad to California, but they would have to get the land first.
War with Mexico first started over boarder disputes but later came about when Mexico refused America's offer of peacefully buying New Mexico and California. When America had captured Mexico City, Mexico signed the treaty of Guadalupe Hidalgo, give America the land and southern boarder they wanted. Because of this war, America now stretched from sea to shining sea.