Complex sentence: Mexico ceded the region to the U.S. in the Treaty of Guadalupe Hidalgo in 1848, except for the Gadsden Purchase of 1853 which added today's southern Arizona and a smaller area in today's southwestern New Mexico.
Split into simpler sentences: Mexico ceded the region to the U.S. in the Treaty of Guadalupe Hidalgo in 1848, except for the area of the Gadsden Purchase of 1853. This added today's southern Arizona and a smaller area in today's southwestern New Mexico.