Final answer:
The clash between Mexican and American soldiers on the Rio Grande over Texas border disputes sparked the Mexican-American War in 1846, prompted by expansionist ambitions of the United States and President Polk's actions leading to the declaration of war.
Step-by-step explanation:
The conflicts over the Texas border led to a clash between Mexican and American soldiers near the Rio Grande, sparking the Mexican-American War. This conflict began in 1846 when the United States, following expansionist desires, claimed the Rio Grande as the southern border of Texas, while Mexico insisted on the Nueces River as the boundary. U.S. President Polk's order of troops into the disputed area led to skirmishes and ultimately to a full-scale war, further fueled by the American objective to acquire more Mexican territories, exemplified by John Slidell's mission to buy New Mexico and California. President Polk capitalized on the resulting tensions and clashes, pushing Congress to declare war on Mexico in May of 1846 following a confrontation that ended with American casualties. These events marked a significant and bloody period in the relationship between the two nations and the history of the American southwest.