During which conflict were American forces defeated in Texas by Mexican troops?

The American forces were defeated in Texas by Mexican troops during the Thornton Affair, which occurred in 1846. This skirmish marked the beginning of the Mexican-American War. The Mexican army engaged American forces near the Rio Grande after disputes arose over the disputed territory. This event escalated tensions between the United States and Mexico, leading to the formal outbreak of war.

The Battle of San Jacinto, on the other hand, was a decisive victory for Texian forces against the Mexicans during the Texas Revolution, leading to Texas' independence from Mexico. The War of 1812 involved conflicts between the United States and Great Britain and is not related to events in Texas. Lastly, the Mexican-American War was a broader conflict that included the Thornton Affair but occurred after this initial skirmish. Thus, the specific answer relates to the skirmish that ignited the larger war.