I'm late, I'm sorry.
In any case, Jedi and first strike conditions are always tricky. The GA got away with it on Coruscant, but they have a heavy NFU base. The SSC got away with the Tygara dom under the claim of slavery while the others had more objectives that didn't include violence. Best thing I can think of here is that some intelligence division warns one of the Light-side factions that there's going to be some sort of ritual on Korriban to either cause some sort of genocide or attack, something that would be a direct threat. Wouldn't even have to be accurate intelligence, governments have attacked for less. Sith are big on rituals, from what I understand of the Primes they could be too.
Or, if you want to pull something galactic-scale, have them doing a ritual that could limit the Light Side in some way. That could lead to interesting reactions from outside viewers who could claim the Jedi only attacked to keep their power base. So on and so forth.