Complex sentence: Early in the Mexican-American War in late 1846, the United States had captured New Mexico and California, and the whole Southwest became U.S. territory upon the signing of the Treaty of Guadalupe Hidalgo, February 2,1848.
Split into simpler sentences: Early in the Mexican-American War in late 1846, the United States had taken control of New Mexico and California. The entire Southwest became U.S. territory upon the signing of the Treaty of Guadalupe Hidalgo, February 2,1848.