What event marked the official declaration of war between the U.S. and Mexico?

The Thornton Affair is recognized as the event that marked the official declaration of war between the U.S. and Mexico. This incident occurred on April 25, 1846, when Mexican troops attacked American troops led by Captain John C. Frémont in the disputed territory between the Rio Grande and Nueces River. The U.S. government, viewing this as an act of aggression, used it to justify the declaration of war, which Congress authorized shortly afterward.

In this context, the other events listed do not serve the function of declaring war. The Alamo was a pivotal battle in the Texas Revolution but occurred in 1836, long before the war with Mexico began. The Battle of Buena Vista was a significant battle during the Mexican-American War but happened after the war had already been declared. The Treaty of Guadalupe Hidalgo was the peace treaty that ended the war in 1848, rather than an event that marked the beginning of hostilities. Thus, the Thornton Affair is the correct answer, as it directly instigated the conflict leading to a formal declaration of war.