The Mexican-American War occurred between 1846 and 1848, with one of the primary reasons being manifest destiny. In 1845, the future American President, James Polk, wanted to acquire the state of California through peaceful negotiations with the Mexican government.
Therefore, he sent his envoy to the Mexican President to agree on the sale of Texas, New Mexico, and California. However, he issued a condition that the total price of this land should not exceed twenty-five million dollars. The Mexican President did not consent to this proposal. Due to this, disagreements between these two nations ensued.
President Polk later declared war on Mexico because he was aggrieved. Both Congress and the Senate voted in support of the war, and this aspect marked the start of the Mexican-American War. The US won the battle, an agreement was signed, and it obtained a considerable chunk of land together with the citizens who were present then.
Additionally, border disputes were another cause of the Mexican-American War. In 1836, Texas gained independence and claimed that River Rio Grande marked its southeastern border. Mexico also claimed that River Nueces marked its northeastern boundary. These stands would later be a possible source of conflict.
Perennial antagonisms between Texas and Mexico resulted in the need for the annexation of Texas to the US. Texas expressed the desire to unite with the United States, and this declaration displeased the Mexican government. In 1845, the US Congress ratified the annexation of Texas, which became the point of conflict between the two nations.
The lack of clear border points became a major problem, and these two nations sent troops to claim ownership of the contested land. In April 1846, the two troops met at the Rio Grande River and engaged in open battle by opening fire on each other, thus marking the start of the Mexican-American War.