Definition of Cabal: The articles or intrigues of a group of persons secretly united in a plot. Or another more simple one: a secret political clique or faction.
What we know of the cabal in game to date is not the Cabal empire itself. Based on grimoire, the cabal we have fought, is a scout team that does not appear to have insight into the why they are here, just orders on what they are supposed to do. Meaning they don’t even know the reasons behind the Cabal proper, the empire giving them their orders. It is unknown how big the inner circle of cabal leaders who are in on the “secret plot” that drives the Cabal Empire.
One thing we do know, is they are military based, and while not necessarily interested in invading us (they might be, we don’t know), they have no issue attacking us on sight without provocation. So without a lot of grimoire basis on why the Cabal are here, we only have what we have seen. So here is what we “know”:
1. No evidence the Cabal were present in our solar system before the collapse. Meaning it appears they came after the collapse. Meaning something happened to bring them here.
2. The Cabal have made no attempt to claim the Traveler. They’ve shown very little interest in the Traveler, unlike the Hive and Fallen who are solely focused on it.
3. The Cabal [i]have[/i] shown interest in both the Warminds and the Dreadnaught.
So why have they shown interest in the Warminds and the Dreadnaught, but not the Traveler? My simple explanation is because they are a war faring species and the Warminds and Dreadnaught are powerful weapons. The Traveler, while powerful, is not a weapon, so they don’t care. They want new super weapons for their war, whatever it may be. Maybe they want them for revenge (grimoire loosely alludes to the possibility the cabal homeworld was destroyed by something; card Cabal 2). Maybe they want to conquer and expand the empire. Either way, they need weapons. Big ones.
In sequence of timing, the Cabal show up on Mars, establish an exclusion zone and start digging and looking for the Mars’ Warmind. I believe what brought them to our system was Rasputin going all “aurora knives” on the darkness with his orbital doomsday weapons (sabre green) which got their attention and interest. A weapon that helped push back the darkness and actually survived the collapse? Who doesn’t want that?
This is what the Psion Flayers are doing on Mars in the Dust Palace strike, looking for the Martian warmind. Read Card Rasputin 2, and watch the Cabal’s reaction when Rasputin blasts them with an orbital strike. They want that shit! Fun side note, we made that happen when we started up the array in Skywatch in the vanilla mission, which allowed Rasputin to gain control of the other warminds. Of course the game does a poor job explaining the significance of that mission, but the grimoire card shows the results which is Rasputin raining death on the Vex and Cabal with orbital strikes on Mars.
So while the Cabal are holed up on Mars digging in the sand for a warmind, Oryx shows up with a brand new superweapon, the Dreadnaught. Ohhhh, what’s that, an even bigger weapon that you can fly around in? So what does the Cabal do? They stop looking for the mars warmind, and instead grab their ship and crash it into the dreadnaught after getting new orders. Primus Ta’aun has no idea why he’s crashing his ship into the dreadnaught, but orders are orders. So he does it, at a great cost. And it should be noted they weren’t trying to destroy the dreadnaught to help us or even necessarily trying to kill Oryx, they were trying to “capture the hive flagship. At any cost.” (Cabal 3). They wanted that weapon, everything else was secondary.
What threat are the Cabal? To date, minimal. I mean, they are a militarized force in our solar system who have no issue attacking us, so that definitely sucks. But they aren’t actively trying to invade Earth or seek out our destruction either. At least not yet. If they got their hands on the dreadnaught or warmind, who knows what they would do. The might go all death star on us and blow earth up (I know Episode VII was cool, but star killer base sounds dumb and I am old school so death star it is). Or they might leave with their new toy and go blow something else up, and never bother us again.
It is over simplifying things, but that is really what I believe the Cabal are here for. Weapons.
But that is only why they are here in our solar system.. Back to my opening sentence, there are secrets to the purpose of the Empire we do not understand yet. The real question is what do they plan on doing with those weapons once they get them?
You say that the Cabal attack us on sight without provocation, but that's not really true; our first interaction with the Cabal was breaking into one of their bases to hijack their communication relays, their initial attack on us was defensive.
Even Zavala recognizes this at the start of the Valus strike when saying "Whether we wanted it or no, we've started a war with the Cabal on Mars."
Aside from that, why are the Cabal in our system? The simple answer is that their empire is fleeing from the Darkness and they are sending scout ships to any system that might provide them with the technology to fight back. They went to Mars in hopes of harnessing the power of the Vex, finding out there was a Warmind there was an added bonus.
After we destroyed the Heart of the Garden, and disabled the Vault of Glass and the Vex conflux network the Cabal lost interest in the now weakened Vex technology and began focusing on taking power from the Hive by crashing into the Dreadnaught and trying to steal it's technology.
Now, the better question is, why haven't the Cabal sent an invasion force to retaliate against the Vanguard? My guess is that the Empire recognized that the both of our forces are working towards the same goal of driving back the Darkness; sending forces to go to full scale war with the Vanguard would ruin any potential chance of an alliance.
I predict that when Destiny 2 launches and the Darkness takes center stage as our biggest threat we will actually join forces with the Cabal to stop our common enemies. This would explain why despite having 5 pieces of content release we haven't had a major conflict with the Cabal.