This site pretty much sums it up: "In 1848, as a result of the Mexican-American War, citizenship was granted to those living in territories gained from Mexico, and while the new citizens gained voting rights, they were aggressively and violently intimidated and the English language was used as a tool to restrict their participation in elections."

In an important, but often forgotten court case, Mendez v. Westminster in 1945 guaranteed the right to integration and equal education in public schools in Southern California, becoming an important precedent for the 1954 Brown v. Board of Education in the African American Civil Rights movement. I was very proud to preside over the meeting of the National Historic Landmarks Committee of the National Park System, several years ago, that resulted in the listing of the courthouse that was the location where Mendez v. Westminster as a Landmark.