Territorial Expansion of the United States.
The United States acquired its first piece of land in 1783; this being the 13 colonies that had fought so hard for their independence during the Revolutionary War. There was a twenty-year period where the country was far too busy establishing itself, becoming stable and putting a constitution into effect to be worried about acquiring new territory. But eventually, the need arose to expand. Under the idea of Manifest Destiny, which was the belief that the United States was divinely mandated to expand from coast to coast, the country began to use any means necessary to expand. Among these were unprovoked wars, purchases from other countries, and treaties.
Before the United States had even considered annexing the Republic of Texas, tensions between Mexico and the U.S. had arisen and become increasingly problematic. When the annexation was approved by Congress in 1845, Mexico cut off all it's ties with the United States. Beyond that, constant disputes over the southernmost border of the newly annexed territory were cause for more trouble in the works. The president of the time, President Polk, was far more ambitious than to just desire Texas; he in fact wanted to eventually acquire all of the land from Texas to the Pacific coast. While Polk did try to avoid violence by sending an Ambassador to the Mexican capital, offering $30 million for New Mexico and California, but the Mexican's would not even see this ambassador, John Slidell. Polk, fiercely determined, send 2,000 troops under the command of Gen. Zachary Taylor to Texas to support the Texans in the border dispute. The Mexican's took this as an invasion of their territory (for the Mexican's believed the border to be several miles further north than the Rio Grande), and engaged the American troops in May 1846. This was what Polk had been hoping for; a valid reason to petition congress for a declaration of war.