Complex sentence: It was intended to replace the existing Dorval Airport as the eastern air gateway to Canada; from 1975 to 1997, all international flights to/from Montreal were required to use Mirabel.
Split into simpler sentences: It was intended to replace the existing Dorval Airport as the eastern air gateway to Canada. Accordingly, from 1975 to 1997, all international flights to/from Montreal were required to use Mirabel.