Question: When was the athletic conference formed that included the 1987 San Diego State Aztecs as a member?

Context articles: San Diego State Aztecs men's soccer
2012 San Diego State Aztecs football team
San Diego State Aztecs baseball
1978 San Diego State Aztecs football team
2014 San Diego State Aztecs football team
1987 San Diego State Aztecs football team
1925 San Diego State Aztecs football team
Western Athletic Conference
1926 San Diego State Aztecs football team
1998 San Diego State Aztecs football team

The San Diego State Aztecs men's soccer team is a varsity intercollegiate athletic team of San Diego State University in San Diego, California, United States.
 The team is an associate member of the Pac-12 Conference, which is part of the National Collegiate Athletic Association's Division I.
 San Diego State's first men's soccer team was fielded in 1968.
 The team plays its home games at SDSU Sports Deck in San Diego.
 The Aztecs are coached by Lev Kirshner.
The 2012 San Diego State Aztecs football team represented San Diego State University in the 2012 NCAA Division I FBS football season.
 The Aztecs were led by second-year head coach Rocky Long and played their home games at Qualcomm Stadium.
 This was San Diego State's 14th season in the Mountain West Conference.
The San Diego State Aztecs baseball team is the college baseball program that represents the San Diego State University.
 Along with the university's other athletic teams, the baseball team became a member of the Mountain West Conference during the 1999–00 academic year.
 Previously, the baseball program competed in the Western Athletic Conference.
 The Aztecs play in Tony Gwynn Stadium, on the SDSU campus in San Diego, California.
 From 2003 until his death in 2014, Tony Gwynn was the program's head coach.
The 1978 San Diego State Aztecs football team represented San Diego State University during the 1978 NCAA Division I-A football season as a member of the Western Athletic Conference (WAC).
 This was the Aztecs' first season in the WAC.
The 2014 San Diego State Aztecs football team represented San Diego State University in the 2014 NCAA Division I FBS football season.
 The Aztecs were led by fourth-year head coach Rocky Long and played their home games at Qualcomm Stadium.
 They were members of the West Division of the Mountain West Conference.
 San Diego State finished the season 7–6, 5–3 in Mountain West play to finish in a share for first place in the West Division.
 However, due to Mountain West tiebreaker rules, because of their head to head loss to Fresno State they were not considered division co–champions.
 They were invited to the Poinsettia Bowl where they lost to Navy 16–17.
The 1987 San Diego State Aztecs football team represented San Diego State University during the 1987 NCAA Division I-A football season as a member of the Western Athletic Conference (WAC).
The 1925 San Diego State Aztecs football team represented San Diego State Teachers College during the 1925 NCAA football season.
 San Diego State competed as an independent in 1925, after having been a member of the Southern California Junior College Conference (SCJCC) since they started playing in 1921.
 They became a member of the Southern California Intercollegiate Athletic Conference (SCIAC) in 1926.
The Western Athletic Conference (WAC) is an American collegiate athletic conference formed on July 27, 1962 and affiliated with the National Collegiate Athletic Association (NCAA) Division I.
 The WAC covers a broad expanse of the western United States, with member institutions located in Arizona, California, New Mexico, Utah, and Washington, along with the "non-western" states of Missouri and Illinois (traditionally associated with the Midwest), as well as Texas (traditionally associated with the Southwest).
The 1926 San Diego State Aztecs football team represented San Diego State Teachers College during the 1926 NCAA football season.
 San Diego State competed as a member of the Southern California Intercollegiate Athletic Conference (SCIAC) in 1926.
 They had played as an Independent the previous year.
The 1998 San Diego State Aztecs football team represented San Diego State University during the 1998 NCAA Division I-A football season as a member of the Western Athletic Conference (WAC).
 This was the last season for the Aztecs in the WAC, as they became a charter member of the Mountain West Conference in the 1999 season.