The CW crossover officially gets going in The Flash Season 3 Episode 8, and it's off to a flying start as Barry recruits all his Super Friends to battle the invading Dominators. This episode, called "Invasion!" after the DC Comics arc, was an absolute delight, seamlessly blending together the three shows set on Earth-1 by continuing plot arcs from The Flash, #LegendsOfTomorrow, and #Arrow.
Flashpoint was neatly dealt with, Oliver missed his chance to capture Vigilante, and Stein discovered that the mysterious woman from his visions is in fact his daughter (and Clarissa's still conspicuously absent. Although #Supergirl was pretty much a guest star, she brought a fantastic dynamic to the team as the perfect counter to Oliver's stoicism and Barry's newfound responsibility. Not to mention, Kara provided some much-needed reinforcements, and it was great to see the Kryptonian hero best every single one of Earth-1's finest.
But after they spent time training together, then fighting each other thanks to the Dominators' mind control, the Super Friends didn't actually make much headway against their alien invaders.
We still don't know why the Dominators are here, or what they want. It seems as though Kara might hold the answer to these questions, as the Dominators attempted to conquer Krypton back in the day (presumably prompting Kara's father to create that alien-busting virus), but so far she's not telling her new friends much. So now it's our turn to do a little investigation, and the comics are the best place to start.
Wiping Out A New Threat
Ultimately, this is the goal of the Dominators, or at least it was in the comic arc Invasion! Forming an alliance of many other alien forces (who seem to be absent in The CW crossover), the Dominators attack Earth in order to destroy all metahumans, as they are the only ones who equal the Dominators in strength and superhuman abilities, and are therefore perceived as a threat.
With the rise of metahumans in #TheFlash, it's certainly possible that the Dominators also want to eliminate metahumans in The CW's Heroes v Aliens crossover. However, it's worth noting that metahumans in the #Arrowverse have barely begun to reach their potential, compared to this point in DC Comics' continuity, when Earth had a thriving population of metahuman heroes.
In the Arrowverse version of this story, perhaps it's humans themselves who present a threat to the Dominators — which would make sense, as the majority of The CW's heroes aren't metahumans. This would also sidestep a potential roadblock for the Heroes v Aliens crossover, as in the original DC arc the Dominators had promised not to kill Earth's human population in order to ensure Darkseid would stay out of their plan. (At this point, Darkseid needed humanity to fuel his anti-life equation.)
Of course, it would be far too complicated to insert Darkseid into The CW's Heroes v Aliens crossover, and by eliminating him The CW have also avoided one of the main reasons the Invasion! arc wasn't wildly popular among fans — there weren't very high stakes in the original story.
So it seems very likely that it's humanity that the Dominators perceive as a threat in this version of the story, specifically the heroes who have already achieved their potential — which would explain why the Dominators' first move was to try and take out the Super Friends by mind controlling them.
But what's the endgame: Do the Dominators want to totally conquer Earth? Again, this seems likely, though they don't seem to have enough forces to take over the entire planet. As seemingly indestructible as the Dominators are, right now they're vastly outnumbered. Of course, in the teasers for the crossover we saw immense ships hovering over Earth's major cities — and Sara Lance seems to be piloting the Waverider away from a fleet of Dominator ships.
All signs seem to point towards the Dominators wanting to conquer Earth because they perceive humanity as a threat to their dominion, probably because The CW heroes are getting too gosh darn powerful and keep saving people all over the place. And we all know how that throws a spanner in the works of all comic book villains' plans!