 After secularization put most California land into private hands, immigrants from the United States began to arrive in steadily increasing numbers, especially in the 1840s when overland routes like the California Trail were opened.In 1848, following the Mexican–American War, Mexico ceded the territory of Alta California to the U.S. in the Treaty of Guadalupe Hidalgo.California was the first portion of the territory to become a state, in 1850.