Yhus a reason... -thinks-
Ding, lightbulb!
There's this magical tool that the light side had, then the dark side stole it but didn't know how to use it so theu went inlitrated the light base and tried to find out how it works. They were found out efore they knew and the spies were executed and the dark side declared war on the light side because they killed a royal who was spying.
Yeah... and that's about all I've got. xDD