Complex sentence: During the Mexican Revolution there was a great migration from Mexico into Southwestern U.S. as thousands, perhaps as much as 10 percent of the total Mexican population, fled the civil war, many going to Texas.
Split into simpler sentences: During the Mexican Revolution there was a great migration from Mexico into Southwestern U.S.: thousands fled the civil war. Almost 10 percent of the total Mexican population emigrated during Civil War years, many going to Texas.