Complex sentence: Historically, teams in one league never played teams in the other until the World Series, in which the champions of the two leagues played against each other; this changed in 1997 with the advent of interleague play.
Split into simpler sentences: Historically, teams in one league never played teams in the other until the World Series, in which the champions of the two leagues played against each other. This changed in 1997 with the advent of interleague play.