This takes place in an alternate universe where Gordon didn't survive the Black Mesa incident, dying while fighting the Nihilanth. He provided enough time for scientists to escape Black Mesa before it was bombed, and hence the resistance was still started by Eli and Kleiner.
It was a quiet day in City 17, the birds were chirping, the striders were roaming, and the metrocops were hunting. It was only the usual for the tired, depressed citizens of the urban center; all of them struggling to survive another day. Though there was one, strange thing. Overwatch soldiers were rushing into the city, pulse-rifles readied, as a strange rift opened in the sky. It was devoid of any light, and an ominous purple glow encompassed its sides, constantly tossing and turning like the sleep-deprived citizens of the urban center. And what came from the rift were large, titanium bullets; descending upon the city like rain in an acid storm. Boom, boom, boom! The ground shook as the pods embedded themselves in the ground, revealing what was inside of them...
The Super Earth Ministry of Science had discovered something ground shaking, an alternate timeline existed past the Meridia wormhole. Furthermore, the Illuminate were using their pagan monoliths to open a gateway into said timeline, though what's past the wormhole isn't what anyone expected. No, it wasn't an alternate universe where the Illuminate won the first galactic war, nor was it Illuminate from a galaxy away; no, it was a gateway into a universe with a regime that took most of the galaxy, including its Earth, the Universal Union. The shadowy leaders of the Helldiver Corps have decided that they will intervene with the union's occupation of our universe, and a new major order was constructed.'
Exiting the pods were men and women in pristine armor, they were the Helldivers. Immediately following their arrival the city was barely quiet anymore, bullets whizzed past civilians rushing for cover as Combine units were taken down one-by-one. Striders being sent to intervene quickly met their end, as the soldiers fired anti-tank rounds into their thought to be impenetrable armor, bringing them down in an instant. Dropships were quickly bombarded by the Helldivers' shots, boom! Boom! Boom! They descended from the sky with a satisfying explosion, their contents deceased.
But the Combine had not even faced the true wrath of the Helldivers, no, not yet. Super Destroyers entered Earth's orbit, supplying the Helldivers with reinforcements and stratagems. The stratagems ranged from simple support weapons to orbital lasers obliterating any enemies that came into their path, all the while Helldivers remained almost high on stims, seemingly invulnerable to all damage - that is until the effects wore off. Explosions and bombs were going off everywhere, exosuits and vehicles were deployed, their occupants running through droves of Combine forces and pulverizing them. It was clear that the Union would need an entire army of reinforcements, and they were soon to deliver.
Meanwhile, squads of Helldivers ran towards the Citadel, dealing with any Combine protecting the way forward. They were going to call a Hellbomb, essentially a mini-nuke capable of obliterating most structures. Though first, they needed to get a hold of certain systems within the Citadel and disable them, giving a clear shot for the Hellbomb. The squad split up to find the Combine terminals throughout the city and wasteland surrounding it. Meanwhile, the civilians still in City 17 were ordered to stay in their apartments and not to exit if they didn't want to be killed, though soon, the Helldivers shall assist them in getting out.