Effects of Annexation--The Mexican-American War

Border/land disputes between Mexican and America were prevalent due to extremely vague Texan borders.
Bloodshed ensued as the Mexican army captured the troops that President Polk sent into the disputed areas.
On May 13, 1846, America declared war on Mexico, and the Mexican-American War began.
America won the war, and with the signing of the Treaty of Guadalupe Hidalgo, not only set firm borders, but also gained more territory including present-day California, Nevada, Arizona, Utah, Colorado, New Mexico, and Wyoming in exchange for 15 million dollars. This is known as the Mexican Cession.

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The Mexican Cession resulted in America gaining California and the entire American Southwest, which make up a huge chunk of America today and helped round out the borders of a transcontinental U.S.
Texas emerged as a major cotton producer in the U.S. It also later dominated the cattle industry and oil industry.
The annexation enabled the U.S. to reach the Pacific Ocean, and in doing so helped America achieve its goal of Manifest Destiny, the belief that it was destined for Americans to expand throughout the continent.
The annexation of Texas cause the Gadsen Purchase which allowed Americans to have more access to the Pacific.
Texas and many of the other ceded lands were heavily dependent on slaves and wanted to be admitted as slave states. This caused disagreement between the North and South on whether slavery should be allowed or not in the newly admitted states or if it should be allowed at all. Disagreements persisted politically in the form of documents such as the Wilmot Proviso, while other disagreements like Bleeding Kansas were more violent. Nonetheless, all of the arguments between the North and South eventually led to the Civil War in 1861.