I am so happy that the new season of Chicago PD is finally here! I've found myself hit with major Halstead/Mouse bromance feels for the past week or so, which is where this fic came into being.

It's mainly going to be me recording my head-canon for this friendship, but there will be a few episode tags and AUs, so if you're into that, read on!

Disclaimer: Chicago PD belongs to NBC and the magnificent Dick Wolf.

"Halstead!" Something - someone? - pounded on his chest, the dull thud echoing through his body. "Come on you bastard, open your eyes!"

Well, since he'd asked so politely… Jay forced his eyelids open slowly and regretted it almost immediately as his already dry eyes were assaulted by thick, irritating smoke. He coughed harshly and blinked, trying to bring the face that hovered just in front of his own into focus. Finally succeeding, he found a pair of intense pale blue eyes staring at him out of a face covered in soot and blood.

Mouse cracked a smile. "Hey, hey, that's good. Ok, man, hold tight. We've gotta go."

Greg Gerwitz, aka Mouse. Tell any one of his high school classmates that the rather eccentric tech geek would end up an Army Ranger and they might have laughed in your face. Gerwitz, go to war? Not on your life.

Jay Halstead had thought much the same when he first met Mouse in training, a skinny pale kid who looked like he'd never voluntarily done physical activity in his life. But Mouse surprised everyone, pushing through training and graduating at the top of his class. When he and Jay were assigned to the same unit shipping out, Halstead couldn't have been more pleased.

And thank god for chance meetings because Mouse Gerwitz ended up saving Halstead's life. Multiple times.

The first time was so routine that it didn't even occur to Halstead until days later. They were on foot patrol when they came under fire. Mouse, the unit's comm specialist, took out a hadji just as he turned his gun on Halstead. The man's body shuddered under the impact of Mouse's bullets, the gun slipping from his grip and his own fire striking the dusty street.

Instead of saying thank you, a week later Halstead got a chance to return the favor. When Mouse mentioned it that night in their barracks, Halstead slapped him on the back and said they were even.

Facing down death next to each other every day bonded the two men fast. They were two boys from Chicago feeling very far from home and the war made them brothers. Halstead was slightly older, more serious, and always ready to stand up for Mouse when the others got tired of his quirks. Mouse was brilliant, sometimes to a fault, full of energy, and ready for anything, especially if it was Halstead's suggestion. Together, they were unstoppable.

It's towards the end of their second tour that they have the incident with the convoy.

The mission was to press forward and join the men fighting on the front lines. It had been a relatively small convoy, just three humvees, with the hope that they would attract less attention. Mouse and Halstead took the lead humvee.

Miles went by with relatively little incident until they hit the IED.

Halstead didn't remember much about the actual detonation. They had been following the road, if you could call it that,a crumbling hill to one side and a dip down the other. It seemed so sudden; they rolled over a slight divot in the ground and a violent jolt had Halstead smashing his head against the humvee's metal interior. When he came to, the humvee was upside down and Mouse was up close in his face, trying to rouse him.

Thompson, their gunner, was dead.

Halstead kept fading in and out, but the next thing he knew Mouse had somehow managed to get them out of their burning vehicle, though things weren't much better out in the open. The rest of the convoy was taking heavy fire from up on the hill and Halstead managed to rouse himself just enough to watch as the third humvee was absolutely demolished by an RPG.

Mouse maneuvered Halstead, who was hammered by a serious head injury and a broken leg, across the street and into the ditch, pressing the taller man's head down into the sand with an order to "Stay here!" before he took off himself towards the remaining humvee. Disoriented, Halstead remained down as ordered until Mouse returned with the three men from the second humvee, the only remaining members of their unit.

It took two hours of staying low and patiently picking off their attackers before the boys of the 75th regiment could finally stop and take a breath. Deciding the demolished humvees drew too much attention, the unit proceeded further down the dip, staying parallel to the road, going just so far as to not be seen from the road but close enough to see any approaching rescue. It took both Mouse and Mackie to move Halstead, who was trying his best not to pass out from pain, but they managed.

That was when Halstead noticed the growing stain of red on Mouse's torso that spread outwards and down on leg of his fatigues.

Closer examination found that a twisted piece of metal from their mutilated humvee had pierced Mouse's side, just above his hip. The bleeding was steady, but not yet life threatening if they got help in the next couple of hours. They opted to leave the metal in.

Besides, Mouse was the only one of them with the know-how to get their comms back up and running. Garcia and Hollinsworth took a trip back to the humvee to retrieve the portal backup radio and Mackie packed gauze around the protruding metal before splinting Halstead's leg as best he could.

Regardless of the fact that he was bleeding out, Mouse's shaking fingers were as nimble as ever. In just 20 minutes he had the comms back on and in another five he'd radioed in to base and received confirmation that an extraction team was on their way.

That was the day Halstead decided that he would take shoot outs or explosions over waiting any time. Waiting was hell. Waiting meant watching Mouse go from alert to slouched over and pale, his ever fidgety fingers still managing to tap out a light beat against his pant leg. They were all still tense and on edge, never completely convinced that they wouldn't be attacked again before the extraction team arrived. With his head wound, Halstead kept fading in and out, but even he was aware that they were low on med supplies, low on water, and low on time.

When evac finally got there, the two Chicago boys had passed out, slumped next to each other on the red-stained sand.

Halstead remained unconscious for almost three days while Mouse underwent a series of surgeries and blood transfusions. When he woke up, they were back stateside. Doctors informed Halstead that he was lucky; he'd suffered no apparent brain injury and his leg would be back in working order after a couple of months of physical therapy. He would recover enough to serve out the remainder of his enlistment.

Mouse came around a couple of days later and although he was still weak and a little pale, Jay was just relieved to see him alive.

Three months later, Halstead was on a military transport back to Afghanistan. But this time he was on his own. Although Jay never actually learned why, in the few days that he had been out from his head wound, Mouse had been medically discharged.