How was it decided California would border Mexico?

The boundary between California and Mexico was established following the Mexican-American War (1846-1848) and formalized in the Treaty of Guadalupe Hidalgo in 1848. This treaty ended the war and resulted in Mexico ceding a significant portion of its northern territories, including California, to the United States. The border was defined largely along the Rio Grande and incorporated existing land claims and geographic features, setting the stage for California's status as a U.S. state.