What event led to the armed conflict known as the Mexican-American War?

The armed conflict known as the Mexican-American War was primarily triggered by the Thornton Affair, which occurred in April 1846. This event involved a skirmish between American troops and Mexican cavalry near the Rio Grande. The U.S. claimed that Mexican troops had crossed into American territory and attacked, while Mexico argued that the U.S. was the aggressor. This incident was pivotal, as it heightened tensions and led Congress to declare war shortly thereafter.

Texas Independence, while significant to the broader context of U.S.-Mexico relations and the war, occurred in 1836, a decade prior to the war's outbreak. The Gadsden Purchase, which happened in 1854, was a later event focused on land acquisition rather than the causes of the war itself. The Annexation of Hawaii is unrelated to the Mexican-American War, as it occurred later in the 19th century and did not have a direct impact on the U.S.-Mexico conflicts of the time. Thus, the Thornton Affair is correctly identified as the immediate catalyst for the war.