Final answer:
A force of 1600 Mexican cavalry crossed the Rio Grande on April 25, 1846, which led to an armed conflict resulting in casualties for the U.S. troops. This prompted President Polk to claim an invasion on U.S. soil, further exacerbating the expansionist-driven tensions and leading to the Mexican-American War.
Step-by-step explanation:
On April 25, 1846, a contingent of 1600 Mexican cavalry crossed the Rio Grande, leading to a clash with U.S. troops. This encounter, which resulted in sixteen U.S. soldiers being killed or wounded, escalated tensions between the United States and Mexico. President Polk declared that Mexico had invaded our territory and shed American blood upon American soil, which set in motion a chain of events leading to the Mexican-American War.
The conflict surged from the expansionist ambitions of the U.S. and disagreement over the Texas boundary. The U.S. claimed the Rio Grande as the border, while Mexico stood firm on the established Nueces River boundary. Infuriated by the attack, Congress declared war on Mexico on May 12, 1846, marking the beginning of a war characterized by high casualties, showcasing the depth of the dispute and making a peaceful resolution seem unattainable.