2001-2002, but here's the best quote from the Wikipedia entry from which I got
that information:
Despite having lobbied to join its in-state rivals Utah and BYU in the WAC for many decades prior to 2005, the Aggies gained membership only after the two other schools had left to form the Mountain West Conference. Later on, Utah State joined the Mountain West Conference in July 2013, again following departures by Utah and BYU.