The U.S. by the 1830's decided that they were ready to expand their borders. However some of the only areas that seemed viable were into Canada or Mexico. Conflicts over the northern border were always a concern and expanding too far in that region didn't seem to make sense. One of the most contested areas was the Oregon Territory which was claimed by both the United States and England. The United States claimed the territory south of the 54/40 Longitude/Latitude.
Republic of Texas - While many in the United States started to settle the California area, another group began to settle in Texas. In the 1820's the government of Mexico had granted a large tract of land to Stephen Austin in order to attract other settlers from the United States. The area began to attract a large number of colonists. Friction existed because of the institution of slavery and the requirement that all citizens of Texas become Roman Catholic. (Mexico had abolished slavery in 1829.) By 1833, there were enough differences that Austin went to Mexico City and began to talk concessions. By 1835, Texans revolted against Mexico. They claimed freedom from oppression, although there is little evidence of the oppression because Mexico had rarely enforced its rules, and the government was inefficient and sometimes corrupt. Mexico sent Santa Ana in to quell the rebellion. While the fighting was going on, Texas voted for independence, and soon the Battle of the Alamo began. The cry "Remember the Alamo," symbolized the valiant fight the 187 rebels fought against the Mexican troops. That they may have had superior weaponry and that perhaps only 179 fought to the death and the others were captured and executed is another part of the story. The reality is that these men fought superior numbers and their story became legend and a battle cry for the war for Teas Independence. Two months later, after another loss, the Texas army under General Sam Houston, surrounded the Mexicans, arrested Santa Ana and forced him to sign treaties to recognize the independence of Texas.
For the next 10 years, Texas existed as an independent republic. Because of the manner it achieved its independence, many thought that immediate annexation was impossible, but it was always on the horizon.
Mormon Trek - This group of religious individuals who went to seek their freedom from prejudice was led by Joseph Smith who in 1830 said that he had received revelations from an angel in a dream in order to establish Christ's Church on earth. He published his revelations in the Book of Mormon which he claimed to be another book of scripture. Essentially, he believed that before Christ another group of Jews made it to the United States where they received the word of God from Christ. Further, this group of Jews were some of the American Indians. His group was determined to recreate this religion and convert the religion. However, most people in the 1830's did not agree with him and they were driven further west. (One of the biggest problems was the group's adherence to polygamy.) When Joseph Smith was killed, Brigham Young sent a group out to Salt Lake to see if it would be habitable for them and then the community moved out to Utah.
The major problem that was had was to determine to whom this territory belonged. Utah wanted to be independent and apply for territory status, the U.S. already believing it had this status, there was an easing of tensions and the region became a territory.
President Tyler meanwhile was pushing for the annexation of Texas and helped a rumor begin that England was going to recognize Texas' sovereignty which quickly guaranteed it territory status.
The annexation of Texas was supposed to be a question for the 1844 campaign. Van Buren was opposed to its expansion and got Henry Clay to agree to oppose it. This angered delegates to the Democratic National Convention so they nominated an outsider by the name of James Polk who not only believed that Texas should be annexed but that claims should be made to the Oregon Territory as well.
Polk narrowly won the presidency. At about the same time, O'Sullivan who wrote for an influential paper defined manifest destiny as "the fulfillment of our manifest destiny to overspread the continent allotted by providence for the free development of our yearly multiplying millions." O'Sullivan not only defined the term, but indicated that it was inspired by God and would mean the occupation at some point of the Untied States to the entire North American Continent.
54/40 or bust - was the rallying cry in order to settle the Oregon border question. The major question was maneuvering the Columbia River. The United States supported a compromise with Great Britain (particularly since they were at war with Mexico and didn't have time to handle two potential wars. The U.S. did get a large natural harbor and Puget Sound which was predominantly all the United States wanted from the area. Meanwhile, those in the north found this acceptable because they realized that if Texas came into the union it would be coming in as a slave state and this region would give some free states to offset that impending reality.
Mexico had accepted Texas' independence but not its claim to the land between the Nueces Rivier and the Rio Grande. When the United States annexed Texas, Mexico broke off relations and prepared for battle. Polk sent troops to Mexico and an emissary to Mexico City to Mexico refused to see the emissary, General Taylor went along to a position along the disputed area. Polk prepared his war message to Congress. Polk would then order other forces to take over Santa Fe. Taylor refused or seemed to refuse to go into Mexico, however the forces in the West were pushing towards California. By 1847, Mexico was ready to sign an armistice. The Treaty of Guadeloupe determined that the Rio Grande would be the southern border between Texas and Mexico and for $15 million the United States would get California and New Mexico. Also people in the Mexican Territories would be American Citizens. The Senate quickly ratified the treaty, gaining California, Utah, New Mexico, Nevada, Arizona, parts of Colorado and Wyoming. Trains who wanted more territory pushed for the Gadsden Purchase which added the southern parts of Arizona and New Mexico.
One thing that is considered controversial is the limitation in the application of the Doctrine of Manifest Destiny because the US. didn't acquire all of Mexico. Part of that was racist in that the US. didn't think they were of the same caliber, and therefore, did not want to include them in the country.
One of the biggest methods of expansion in the country was the railroad. With it, people and goods could travel across the country. So could the slew of inventions and innovations that became prevalent in nineteenth century America.
With the expansion of the country, the development of inventions, industrialization began to grow. With the need of more and cheaper labor, immigration became a more important ideal. People from other countries would tolerate poorer conditions and worse pay than American citizens.