What event marked the beginning of the Mexican-American War?

The beginning of the Mexican-American War was marked by the attack on American troops by Mexico. This pivotal moment occurred in April 1846 when Mexican forces engaged U.S. soldiers in the disputed territory between the Rio Grande and the Nueces River. The conflict escalated after President James K. Polk claimed that American blood had been shed on American soil, leading Congress to declare war on Mexico shortly thereafter. This military engagement created the spark for the larger conflict and established a context of tensions over territorial disputes that had been brewing for years.

In contrast, the other options—such as the Declaration of Texas independence or the Annexation of Texas—are significant events that contributed to the rising tensions between the United States and Mexico, leading up to the war. However, they did not directly trigger the hostilities. The settlement of the Oregon territory was unrelated to the Mexican-American War, focusing instead on U.S. claims in the Pacific Northwest and not involving Mexico directly. Therefore, the attack on American troops represents the immediate catalyst that ignited the war.