When did the conference the members of the Fresno State–San Diego State rivalry are both part of begin operations?
Fresno State–San Diego State football rivalry: The Fresno State–San Diego State football rivalry is an American college football rivalry between the Fresno State Bulldogs football team of California State University, Fresno and San Diego State Aztecs football team of San Diego State University. Fresno State–San Diego State football rivalry:  Both schools are members of the Mountain West Conference. Mountain West Conference: The Mountain West Conference (MW) is one of the collegiate athletic conferences affiliated with the National Collegiate Athletic Association (NCAA) Division I Football Bowl Subdivision (FBS) (formerly I-A). Mountain West Conference:  The MW officially began operations in July 1999.
July 1999