Disclaimer: I do not own FMA or any of the characters.
Looking back, she couldn't tell when the line had been crossed. She could find memories of times when it had not been crossed, times when the moment had already occurred, anything but the moment itself.
When had it happened?
After her father's funeral was too early. Then she'd been nothing more than a naïve little girl, bewildered by the prospect of joining the world, searching for something - anything - stable. He'd fed her his dreams in all their untested glory, and she'd seen something she wanted a part of. That had been all. She had simply wanted to do some good in the world.
Abandoning her post had only been possible because the moment had passed so far back. It was nearly possible to forget what her mindset had been like even a year prior, when she couldn't have thought that she would ever take such an action. She'd taken pride in that uniform, in climbing the ranks along with him- but it was a hindrance now, and he needed her elsewhere. Her famous skills were crucial to the mission, and her uniform would serve no purpose. It could be discarded; only with ease because it was clear it would serve his purpose best.
Ishbal then. No, still too early. She had been too busy staying alive, keeping other men alive, perfecting every single lethal shot, destroying her naïveté with every one. With every step she was a killer. It was then that she shut herself down, twisted herself into a machine, allowing herself to feel nothing other than calculated logic, dreaming of nothing other than the end of the war. Her duty to country had been enormous, taking lives because her orders said to. It had only been in private, or in front of him, that she'd allowed doubt to creep in, wondering why the innocent dream of helping those who had no help had to have been thus shattered.
The reassignment was far too late. By then she knew what her true duty was, and it was no longer what she had pledged to her country. That was treason for a soldier, of course, having a greater allegiance than the one professed to one's country, but it didn't matter. She was too far in, too far gone to care. Buried between logic and strategy was the hidden truth of why she was willing to serve the Fuhrer. Why she was willing to put her life at stake for just a little more intel, or the opportunity to find weakness.
It was tempting to place the line at the moment when she was assigned to serve as Colonel Mustang's aide. It was tempting to put it at the moment of that fateful promise. But that wasn't quite right. It was the moment when she accepted the definite change in who they were to each other; she was no longer her father's daughter, she was a hardened soldier, with ideals and goals now tested by hardship. And he was no longer her father's student, someone she had trusted to make the best decisions with her father's research. He was her superior officer, and it was her duty to protect him. She would have protected any superior officer. The promise tied to her ideals; keep on the straight and narrow path. But she would have pulled the trigger had he failed, and betrayed the trust she'd given him.
Sometime in the years of being his aide, before Edward joined in the chaos of the office, the line had been walked. Something in the normalcy that was the Colonel's consistent lack of finished paperwork, the banter between the boys, and serving as the only real organizational force, she had found a home that was more comfortable than her own apartment. And sometime then, she realized that he would never need her to uphold her promise; the mere fact that it existed was enough to keep him on track.
It must have been sometime after this whole mess had started that she'd definitively crossed it. She knew she'd crossed it after that fateful fight against the homunculus Lust- her lowest moment as a soldier, a person, a woman, a fighter. The scolding she'd had after that had been completely merited, and she'd tucked the thoughts that niggled at the edges firmly down, away, out of her thoughts. She had a duty to do- even if she was barely aware that that duty had shifted ever so slightly over the years, and now meant something other than her issued orders.
She'd hoped to never need to voice it, like she'd hoped that she would never need to act on her promise. She'd hoped that she could see the signs before it became that dire, nudge him back before he went wide of his mark. But when he lunged sideways, it was all she could do to catch him.
Her hands had shaken. In all her years of death, this would have been the one unlivable offense. Every single life she'd taken was unpardonable, but the soldier's burden was to shoulder it. Not this one.
When had she realized that if she had to kill him, she would finish what he had started and then cease to live? When had she realized that he was more precious to her than the goals she'd treasured so closely? When had the line been crossed? The exact moment, the precise hour and day when her goal had changed from "Help him change this country" to "Protect him. Protect him so he can change this country". When had she allowed something deeper than friendship to change her goals?
When had she realized that she would not live a life without him?