Final answer:
In July 1845, President James K. Polk ordered General Zachary Taylor into the disputed territory between the Nueces River and the Rio Grande, setting the stage for the conflict that led to the Mexican-American War.
Step-by-step explanation:
In July 1845, a significant event leading up to the Mexican-American War involved U.S. President James K. Polk ordering General Zachary Taylor to move into the disputed territory between the Nueces River and the Rio Grande. This provocative action laid the groundwork for the skirmishes that would follow. Polk seized this moment to provoke war with Mexico and after the outbreak of hostilities between General Taylor's forces and the Mexican troops, Polk declared war on Mexico, framing it as a defense against Mexican aggression. Mexico had a different view, considering the U.S. actions as an invasion of their territory given the longstanding dispute over the Texas border. The U.S. annexation of Texas in 1845 and subsequent military actions were driven by the expansionist agenda to extend U.S. borders to the Pacific Ocean.