U.S. States That Were Once Part of Mexico

Mexican Cession (1848) + Texas Annexation (1845)

Eight present-day U.S. states were fully or partially Mexican territory before the Mexican–American War (1846–48) and the Treaty of Guadalupe Hidalgo (1848):

Additional Notes

• Oregon, Washington, Idaho, Montana, Wyoming were disputed with Britain (Oregon Treaty 1846), not Mexico.
• Gadsden Purchase (1854) added southern AZ/NM for southern transcontinental railroad route.
• Mexican land grants (ranchos) in California, NM, TX were supposed to be honored (often weren't).