I'm a little confused about the EDF games storywise. Is Global Defence Force, Earth Defence Force 2017, Earth Defence Force - Insect Armageddon, etc. stand alone or are they connected storywise?
The EDF games barely have a story at all. People shoot alien bugs is about it.
EDF 2017 starts with human's first contact with aliens (including a great "We're calling them Ravagers. We hope they're friendly" bit), followed by them invading in 2017. Seeing as how you fight aliens in the previous games, it's obviously not a direct sequel.