A relatively common way to start wars since the dawn of Chaos has been to make up some offense, attack, or otherwise SLIGHT as a casus beli. Like jon says, I think its a great start that allows overall faction narrative to contribute to the end of the thread. What gets lost in a lot of invasions threads I think is that these threads are essentially battles, one step along the way of the narrative that ultimately gains the winner the hex (or neutralizes it) and that these are also likewise, a part of ongoing wars and conflicts that aren't just focused on the planet where all of our characters are when we write.
But maybe then, instead of a 5th permanent category, maybe shift it towards this idea becoming its own Mandate? if only because if a faction has the new Fleet mandate and this went through that's six factors for judges to take in. This way also the weakness for defenders isn't as much of a concern