The debate over whether America really had a manifest destiny to expand all the way west or if it was used as an excuse to acquire more land led to debates in U.S. politics. Advocates of manifest destiny, mostly democratic, argued that the U.S., as a more advanced culture, had a God-given right to expand its borders. They believed the expansion would civilize the West and Americas democratic, cultural, and religious values would benefit the Native Americans. In addition, supporters would argue that the belief would strengthen the union, making it invulnerable. On the opposing side, consisting mostly of the Whig party, the God-given right to expand all the way westward at the price and rights of thousands of innocent natives was blasphemy. The Whig party was not manifest destinies only critic, abolitionist, fearful of slavery spreading, argued that the constitution did not give the country the right to gain new land and the countrys vital institutions would suffer as America was spread too thin.
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Texas sought to join America as a new state, after it gained independence from Mexico and had a revolution. The process of expansion in which newly democratic and free states would seek entry into the United States, rather than the U.S. extending its government over unwanting people was ideal. The Democratic Party was threatened to fall apart if Texas entered the Union, as it would become another slave state and this forced both Presidents Jackson and Van Buren to decline Texas plea.
During the election of 1844, both Henry Clay of the Whig Party and Van Buren of the Democratic Party were against the annexation of Texas, this displeased the Democrats as they wanted to gain Texas so they dropped Van Buren in favor of James Polk, who was for adding Texas as another slave state. Polk cleverly tied Texas annexation into the Oregon dispute, the controversy over Oregons border. In 1846 the dispute was settled over the Oregon Treaty where the British relinquished its holding to the lower Colombia basin. This appeased expansionist in the north, who fought for Oregon and expansionist in the south, who focused primarily on Texas.
After Polks election, he moved to occupy a free portion of Texas that was still claimed by Mexico. This sparked the Mexican-American War in 1846, were there were calls for All Mexico, mostly from Eastern Democrats, however Mexicos annexation brought up much debate. If Mexico were to become a part of the United States it would mean millions of non-white Mexicans would become U.S. citizens, something Americans were not too keen on. The racist aspect of Manifest Destiny considers inferior Mexicans unqualified to become Americans whereas the mission aspect of Manifest Destiny dictates that Mexicans would become improved under American democracy. The All Mexico movement quickly abated with the Treaty of Guadalupe Hidalgo in 1848 which granted Alta California and Nuevo Mxico to the United States, both of which were sparsely populated with Mexicans.
After the Mexican-American War ended in 1848, disagreements over the expansion of slavery made further annexation by conquest too divisive to be official government policy. The belief in Manifest Destiny in the 1840s greatly influenced both U.S. politics and policy and is to blame or thank for Americas expansion from sea to shining sea.