Commissioner Fay Vincent orders realignment of the National League for the 1993 season, forcing the Cubs and Cardinals into the Western Division. The two teams will oppose the move, and the NL will split into three divisions in 1994.

On July 6, 1992, Commissioner Fay Vincent orders realignment of the National League for the 1993 season, forcing the Cubs and Cardinals into the Western Division. The two teams will oppose the move, and the NL will split into three divisions in 1994.

Source:

Baseball Reference July 6