Final answer:
In January 1846, U.S. troops building a fort on the Rio Grande clashed with a Mexican cavalry, resulting in casualties. President Polk cited this as an invasion, leading Congress to declare war on Mexico, marking the start of the Mexican-American War over expansionist disputes.
Step-by-step explanation:
The events in the Nueces Strip in 1846 were integral to the lead-up to the Mexican-American War. Following President James K. Polk's orders, U.S. troops moved to the Rio Grande to construct a fort. A clash with a Mexican cavalry patrol ensued, resulting in the deaths and injuries of sixteen U.S. soldiers.
This incident, coupled with the longstanding border disputes between the Nueces River and the Rio Grande, provided President Polk with the pretext to push for a declaration of war against Mexico. Congress granted this declaration on May 12, 1846. The roots of the conflict lay in the expansionist ambitions of the United States, driven by a desire to acquire more Mexican territory. The Mexican-American War became a pivotal chapter in U.S. history, shaping the nation's territorial expansion and influencing its relationships with neighboring countries.