The election of James K Polk was the most significant change in the 1800's
When James K Polk was elected he set out to gain the Oregon Territory and South-Western Mexican Territory. He was the Leading presidential democratic candidate and was elected.
James Polk (1795-1849) served as the 11th U.S. president from 1845 to 1849. During his tenure, America’s territory grew by more than one-third and extended across the continent for the first time. Before his presidency, Polk served in the Tennessee legislature and the U.S. Congress; in 1839 he became governor of Tennessee. A Democrat who was relatively unknown outside of political circles, Polk won the 1844 presidential election as the dark horse candidate. As president, he reduced tariffs, reformed the national banking system and settled a boundary dispute with the British that secured the Oregon Territory for the United States. Polk also led the nation into the Mexican-American War (1846-48), in which the United States acquired California and much of the present-day Southwest. Polk kept his campaign promise to be a one-term president and did not seek reelection. Soon after leaving the White House, he died at age 53.
When Polk set out to accomplish his goals he would have to fight Mexico for the South-Western Territory and had to "negotiate" with Britain for the Oregon Territory
Polk knew Mexico would not hand over the territory so he would have to fight for it. In to the War
After nearly two years of fighting, peace was established by the Treaty of Guadalupe Hidalgo, signed on February 2, 1848. The Rio Grande was made the southern boundary of Texas, and California and New Mexico were ceded to the United States. In return, the United States paid Mexico the sum of $15 million and agreed to settle all claims of U.S. citizens against Mexico.
Negotiations between the United States and Britain over the Oregon Country began in the summer of 1845. The initial American proposal called for the boundary to be drawn at the 49th parallel, bisecting Vancouver Island. When British negotiators rejected this proposal, President Polk took a bolder position by reasserting his campaign promise to support the 54° 40' line and announcing the American intent to terminate the joint occupancy agreement within a year. While expansionist Northerners cheered these provocative actions with shouts of "Fifty-Four Forty or Fight!," Southerners in Congress made it clear that they would not risk war with Britain over Oregon. British leaders were similarly adverse to conflict and did not want to jeopardize their important economic relationship with the United States. In June 1846, the Senate, preoccupied with war against Mexico, quickly approved the Oregon Treaty with Britain, setting the boundary at the 49th parallel.
Polk's fight for the territory ended up with The Oregon and South-West mexican territory as you know. The fight for the Mexican Territory was not easy, many died in the fight; in fact they lost the first fight for the Alamo.
The gain of these new territory's meant that this land was now a part of the U.S. This meant that the U.S now spanned the entire continent and they had accomplished part of Manifest Destiny. It meant that this land was a completely new opportunity for all those who did not want city life or wanted to escape your situation. It meant people from the North or the South could start anew. Slaves could escape and lead a new life.
This increased westward expansion very much. President Polk's goal as president was to expand westward, the very definition of the phrase. If anyone wanted to leave the already settled place the new west was completely open. The common man could build an empire. Homesteaders could be the leaders of their own life.