Original masskink prompt was:

"basically, for whatever reason, shit hits the fan and both garrus and shepard end up in the middle of the first contact war after going through the Omega 4 relay. Cue them trying to negotiate peace from each of their respective races. I would like them to be in an established relationship already, just to see how they'd handle their relationship in the face of such fear and mistrust the turians and humans had with each other.
Do they hide their relationship or use it to show how wrong each side is about each other?"

and this is what it turned into...comments are highly appreciated.

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Shepard came awake to the sound of gunfire and explosions, tasting dust in her mouth. Instinct took over as she rolled to her side, unclipping her assault rifle to face whatever new challenge would present itself. It took her a few moments to understand that the sounds of battle were a bit distant, and there was no immediate threat to respond to just yet.
She marked the direction the noise was coming from, just as it faded, identified the low hum of some sort of engine that was kicked into gear then moved away into another direction until if faded, too.
Whatever that had been, it hadn't been about her, but that didn't mean she could keep just lying there. However much she'd have liked to continue doing this at the moment.

A groan and a familiar shade of blue in her peripheral vision let her know that Garrus was present and presumably in about the same state.
"What the hell?" came his voice, sounding just as disoriented as she felt.

She could echo that sentiment. The last thing she could remember was clinging to the door frame of the Normandy's cockpit as they raced for the Omega 4 relay, the damaged ship shaking and bucking like a hurt, living creature as the Collector station behind them blew into pieces, EDI giving damage reports that no one save the AI itself was interested in hearing about at that point. And there had been something about an anomaly, but given the circumstances she hadn't paid much attention.
If she was completely honest she hadn't thought they'd make it.

This, however, wasn't the Normandy.
They were planetside, by the looks of it in some moderate natural setting, dirt ground beneath them, large specimens of local vegetation obscuring the view towards where Shepard assumed the fighting had happened just minutes ago. Their surroundings looked rather undisturbed by civilisation.
The sky overhead was a disturbingly normal and serene blue, and, for the moment at least, clear of any aircraft and therefore threat to them.

With some effort, she climbed to her feet, trying to do so quickly before her body caught on to it and responded with more than the general soreness and exhaustion that she already felt. Garrus did the same, moving just as stiffly, and she gave him a quick look.
"You all right?"

He snorted. "Let's settle for not significantly worse off than when we got to the Normandy. Now, where the hell are we, and how did we get here?"

"Let's find out." Shepard fired up her omnitool. It should at least be able to patch into the normal communication feed that was a standard across the known galaxy, a permanent data stream for providing at least basic information as time, location and rudimentary navigation.
She came up blank, which was unusual, because usually even the smallest, most backwater colony had something of the sort, and was about to complain about it when Garrus suddenly snarled in alarm and reached over, turning her omnitool off.

She was about to at least make a sharp remark about that when he proffered his own lit omnitool, showing her location and date, and her protest died on her tongue.
"Shanxi? But that's not-"

"No. Look at the date."

She did, and her skin went cold. "That's impossible. This must be some prank, or malfunction."

"I don't think so." His tone was odd. "You didn't get any feed because there is no public one available. I'm getting one that's on a restricted frequency. The encoding's decades old, but it's Hierarchy military."

"You mean to tell me the year is 2157 and we're on Shanxi?" She had meant to make it sound like a bad joke, but she didn't quite succeed, as cold dread started to settle in her stomach.
This place, at this time, would be the stage for the bitter conflict that had followed the first encounter between their two respective species. Something that had started out as a misunderstanding had turned into a three-month conflict that had soured relations between turians and humans for more than three decades.
And they were right in the middle of it. Shepard very briefly considered whether they indeed hadn't made it back through the relay, she had died again and this was hell, then dismissed the possibility out of principle.
"Not possible," she repeated, even if the environment around seemed hell-bent on proving her wrong.

Yet she was already on her way to investigate.

They cleared some larger vegetation that had apparently kept them from being detected, and came upon what was obviously the scene of the conflict she'd heard minutes ago.
There was a burned spot on what looked like a rough road, with tracks made by wide tires leading through it. Some fragments of warped metal were strewn about. There were footprints of both humans and turians.
She counted five human bodies on the ground, three men, two women, all of them in civilian clothes, all of them rather young, all dead by gunshots. None of the bodies had any weapons left, but that really didn't come as a surprise.
She checked each of them, identified some tattoos on three of them that wouldn't make sense to a nonhuman, but were quite common among Alliance soldiers. None had any identification tags, however.
There were spots of blue blood on the rough road, some of them substantial, and drag marks around them.
Shepard grimaced.
"They set an ambush for a turian patrol, some explosives, maybe a mine to stop their vehicle. Didn't work out, apparently. So the patrol killed them, took their weapons, then continued on its way and left the bodies here."

Garrus visibly hesitated, trying to gauge her mood, but then pointed at the bloodstains. "The patrol took some losses, too. They dragged them into the APC." He hesitated again, then added "Wounded or dead are never left behind if there's any other choice."

"Well, that's supposed to be true for Alliance military, too." She debated with herself for a moment, then quietly said "Those were Alliance military, not civilians. They didn't wear uniform or carry dog tags, but I recognised some of their tattoos."

Garrus didn't seem surprised. "So they camouflaged themselves as noncombatants."

She shook her head. "That's not...this is against the rules of war even at this time. This is wrong. All of this is."
She couldn't articulate it, not properly, but she felt sick to the stomach about this whole mess. She had been in the military for about half her life, and she fought and killed enemies on an almost daily basis, but this was different. These weren't pirates, raiders or other criminals. There were regular soldiers on both sides of this. That made all the difference, because she had never before been in quite that situation. Even in the conflict against the batarians in which she had fought, their opponents had been criminals, not regular soldiers or even militia. It didn't matter that these criminals' actions had very likely been sanctioned by the batarian government. In the end, it just had been another fight against criminal elements, however well equipped.
In all the conflicts she had fought, she had been able to tell herself that she was doing the right thing, fighting on the right, the just side, for the right cause. There were some decisions that weighted on her conscience, true, because armed conflicts never were clean-cut, ordered affairs where one could be certain to only ever precisely target the real enemy instead of any civilians, bystanders and generally desperate, scared people that happened to be in the wrong place at the wrong time, but on the whole she had never regretted being Alliance military. Unlike this here. There was no justice or honour in this. This was senseless, a conflict that served no purpose save to ruin relations between their respective species for decades.
The turians here viewed the humans as criminals, for breaking the law of the galactic community, and in their own way only did the duty they had been sworn to. The humans only knew that they had been attacked for no apparent reason at all, by aliens that matched the definition of monsters quite well. Well, she amended, at least until humans met their first krogans, at which point the scale would have to be adjusted a bit. Humans tended to go too much by outer appearance still, and even in her own time there still was a lot of mutual distrust between humans and the rest of the galactic community.
And all of that because of some damned misunderstanding, the actions of damned, stubborn fools on both sides. Then again, that probably held true for most wars.

"Is there anything you want to do for the fallen?" Garrus asked quietly. "I'm not familiar with human procedure on this, but-"

"No," she interrupted. "We're in hostile territory, and at risk ourselves, and it's not feasible to give them any sort of burial. There are no tags for me to collect and to return to their CO either, so there is nothing I can do here and now." She turned away, hating having to do this despite her words, because this wasn't right either. They had been fellow soldiers who had fallen in the line of duty. They deserved some sort of acknowledgement, and she had none to give. "Let's gain some distance from here, before anyone comes back to check. We can't afford to be caught by either side."

He nodded in agreement, not even voicing the question that had to be on his mind as much as on hers. What do we do now, and how the hell do we get back?

She was glad for that, because she had no answer to that one. But first things first. Right now they needed a reasonably safe place to rest up a bit. She hoped that by then she would have at least some vague idea as to how to get them out of this fix.