The Democratic Republic Of Bineeks was waging war against the Unified Ceefar Electorate. The TGC had no stake in such a conflict, other than being slightly amused at the name of the first. Taylor Huce was pretty much the most non-threatening person to exist, a tiny shy brunette with a bit of social anxiety. She was unexceptional. At the time she was on vacation with her significant other, Tr'Boke Halkc, on the planet of Lori Iv in the Electorate's territory, a newly colonized planet of only a bit over a billion people, and was of no military worth whatsoever.
The two civilizations had only known each other for a short time at this point, but Taylor and Tr'Boke had known each other more or less that entire time and cared deeply for each other. He was good for her mental health. He made her laugh, made her feel like perhaps she didn’t always have to be on guard.
Through a chain of miscommunication and incompetent leadership, Lori Iv was mixed up with a weapon-manufacturing ecumenopolis. A fleet was dispatched to bombard the planet. To the people manning the fleet, it was pretty obvious that the planet was not in fact a manufacturing complex, but to question orders was to invoke the wrath of the inquisitors, so screw that.
The planet had some standardized defenses, but they were only prepared to fight off a bit of piracy, not an actual fleet of warships from the Republic. The planet's defenses were quickly overwhelmed and nuclear weapons blanketed the surface. Taylor received third-degree burns, internal bleeding, and a nasty blow to the head. Tr'Boke wasn't human. He was a bit less durable. When Taylor came to a few moments later, her nanites already gathering resources from the surroundings, her loving boyfriend, her boyfriend that made her laugh and smile, was dead. Not just dead, but mostly mush. The Republic fleet left, their mission accomplished.
Taylor's immediate response was a bit of hysterical laughter. Her second response was to carry the larger solid section of her boyfriend's corpse back to their ship. The ship wasn't directly targeted by the orbital bombardment so it was fine. Its surroundings were less so. She had turned off its proactive defenses to prevent massive collateral damage were someone to toss a rock at it. She desperately wished she hadn't. By giving a damn about collateral damage she had prevented the ship from automatically shooting down the nukes. By giving a damn about collateral damage she had inadvertently killed her boyfriend. What she did next was maybe a bit of an overreaction, but suffice to say she was a tad bit irked at the Republic. Her minor bit of brain damage wasn't a priority. It surely would in no way influence her emotions or decision-making.
She had her ship's fabricator start making mining drones, there was plenty of scrap around after all. The mining drones brought back materials, and the fabricator started making more fabricators. A short while later and she was stripmining the planet. Once the materials available on the surface were collected she started setting up deeper drilling operations and took her ship to orbit. The fabricator on her ship started making more fabricators, only this time with coilgun-delivered resources. About twelve hours after her boyfriend was killed she was mining away at every planet and moon in the system, the drones battery-powered, the recharging hubs spread throughout the system each with a bit of antimatter fuel from her ship. Of course, from her perspective, it was more along the lines of twenty-four hours since her perceived seconds per second was at two. More than enough time for resentment to simmer.
She attached cargo sections to her ship and collected excess processed materials from each planetary manufacturing hub. Eighteen hours after the attack she finally had enough materials to start creating a Dyson swarm, though just start of course. Even just a minuscule portion of a portion of one provides quite a lot of energy. Lackluster compared to a proper ring group, or even just a more completed swarm, but it'd do. Fabricators formed an expanding perimeter as the swarm grew. Twenty-four hours and she had enough power and materials to start making stellar lifters, harvesting vast amounts of plasmic hydrogen and helium with, relatively, sparse bits of other elements mixed in.
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Thirty hours post-attack and she was able to start creating more exotic materials. Thirty-six and FTL cargo ships flooded the star system. Fourty-two and she had constructed a basic gateway and synced it to the rest of the network, a time a fair bit shorter than had she made the trip back to human space the normal way.
Fourty-eight hours and drones of much higher quality were being sent outward from the border of humanity's territory not facing the Union. Not von Neumann probes though, those were rather outdated after all. Von Neumann probes require each probe to have manufacturing capabilities and hefty computer systems. All a gateway drone needs is a big engine and a big warp drive. Once at the destination the drone detaches from the folded-up gateway it was hauling, and the gateway unfolds into a functioning connection to every other gateway on the network. The drone goes through to collect another gateway and more drones head through it to every other nearby star that doesn't already have a drone en route. Once that's completed a bigger folded-up gateway is sent through the first. Then a bigger one is sent through that one. A few iterations of this and the star system is capable of handling regular traffic, starting with the beginning of construction of a Dyson ring group.
Seventy-two hours after the Republic doomed itself she started sending automated warships back through the portal in Electorate space. The Electorate was for the most part just grateful for the support against the aggressors.
The first battle against the Republic ended quickly. Boarding pods full of robotic soldiers and toxic gas meant the ship was still in decent condition. Easier to reclaim materials, and more importantly information, from a ship disabled by chemical warfare than antimatter ammunition. Taylor wasn't a chemist, but bored people on the internet quickly provided a tailormade chemical formula targeting the handful of species of the Republic. The TGC looked into things of course, but the chemical weapons in question were designed to kill quickly, not cause unnecessary suffering, so all was fine. They hadn't received a formal request for aid, so what someone did outside of their territory wasn't their problem.
Skipgates shove things into warp, which can be used to give ships a headstart. On the other hand, when something without a warp drive is launched via electromagnets or gravity manipulation then skipgated, the object will decrease in warp factor until it hits one, at which point the object is in the same plane of existence as everything else not in warp. The entire time it's in warp the velocity relative to warp one is multiplied by the warp factor. Aimed carefully, a pizza can be delivered right in front of you, surrounding walls completely ignored. Aimed carefully, a capsule made of regular matter, containing antimatter via electromagnets, can be dumped right into the middle of anything which doesn't have anti-warp defenses. And, really, who would put anti-warp defenses on a planet when the gravity well of the planet takes care of such just fine?
When Taylor started deleting planets in the Republic's territory the TGC looked into things again, but she was acting on behalf of herself, not a corporation, so it wasn't the TGC's problem.
Taylor wasn't a biologist, but bored people on the internet quickly provided a tailormade virus targeting the couple of species of the Republic. Bioweapons are by their nature hard to control once used, so the TGC looked into things, but she had distributed a counter-plague to the polities surrounding the Republic, so those of those species outside the Republic wouldn't be affected.
All in all, it took seventy-eight hours for the situation to go from Taylor having only a ship to the Republic being completely outmatched. It took ninety hours for the republic to collapse into civil war. It took one hundred and eight hours for the republic to no longer exist.
Everyone not completely braindead learned two lessons. One: make sure there aren't any humans where you're aiming. Two: if a single civilian can do that, it’s probably best not to mobilize the TGC as a whole against oneself. Of course, not everyone actually got the message, but idiots'll be idiots.