Which boundary was defined at the end of the Mexican-American War?

The boundary defined at the end of the Mexican-American War was the Rio Grande. The war, which lasted from 1846 to 1848, culminated in the Treaty of Guadalupe Hidalgo, in which Mexico ceded a significant portion of its territory to the United States, including what is now California, Nevada, Utah, and parts of Arizona, New Mexico, Colorado, and Wyoming. The treaty established the Rio Grande as the border between Texas and Mexico, which was a significant aspect of U.S. expansionism and the concept of Manifest Destiny during this period.

Choosing the Rio Grande as the correct answer also reflects the geographic realities influenced by the conflict, with the river serving as a natural boundary that has been politically and culturally significant in U.S.-Mexico relations ever since. The other rivers listed—like the Mississippi, Ohio, and Missouri—are important geographical features but do not pertain to the territorial changes or boundaries established as a result of the Mexican-American War. Instead, they relate to earlier historical contexts or different regional dynamics within the United States.