The Lannister army had been marching for two weeks from King's Landing with Jaime at the head all because some of old Walder Frey's idiot sons had let the Blackfish steal a castle from them. He was sick of fighting other people's battles. Duty to the Lannister bannermen required this of him, duty to his son - the King - had required this of him, and duty to Cersei who was about to stand trial in the capital had required this of him. So much duty, he mused, for so little reward.
He was sure that this was just a distraction orchestrated by the High Sparrow - a way to keep him out of the way during Cersei's trial. As her brother and the head of the Kingsguard, his place should have been by her side, but the King had been very firm, stripping the knight of his position as head of the White Cloaks and sending him off to the mud-infested river lands. He knew that Cersei was to blame in the end, for her troubles as well as his - Tommen had been in her twisted grasp for too long and he had taken on too many of his elder brother's mannerisms of late, not to mention the High Sparrow's influence as instigated by the girl Margaery. Jaime wanted to save him from all that, but he feared it was too late now. He loved his sister, but he was not so blind as to ignore the effect that she had on anyone she dug into - including himself, not anymore.
All that you rely on
And all that you can fake
At least Jaime was not alone. On top of the 2000 men he was leading, he'd convinced his steward to come along, and he had Bronn. That was something. The former sellsword was full of complaints and vocal disdain for anyone but himself at all hours, but he and Jaime got along well enough; Bronn had helped Jaime train his left hand with the sword, and they shared an affection for his younger brother Tyrion whom Jaime had helped escape following his disastrous trial by combat. Cersei still believed that Tyrion had killed Joffrey, but Jaime knew Tyrion well enough to know that, had he been the one to murder a son of the brother he loved, he would have admitted it.
Joffrey had been hateful (this Jaime freely admitted despite having been the young King's true father) and had been especially hateful to Tyrion and Tyrion's wife, Sansa Stark. Cersei had done nothing to discourage the boy - quite the opposite. She seemed to thrive on his cruelty as much as he did. When Joffrey was killed, Cersei had called for violence. Sansa had suspiciously enough made her escape even as Joffrey lay convulsing in his mother's arms, and Cersei was offering a knighthood to whomever found her and dragged her back to the capital for punishment. But Jaime wasn't convinced of Sansa's guilt either. She wasn't a killer - yet. Truth be told if she had not escaped when she had, Jaime would have been bound by his oath to find a way to smuggle her out of the capital before harm could come to her - he would not be seen breaking that oath, especially not with his tall towheaded plank of a conscience lurking.
Brienne had been there that day at the bride's invitation. After wishing the newly wedded couple happiness, she had been cornered by Cersei. He'd caught the sight of them standing together and his stomach had sank. He'd not shared anything about Brienne with the queen beyond the fact that she had been sworn to Renly, then Catelyn, and finally tasked with bringing him home, and that she had obviously done so successfully.
He'd never told her about the fever taking him and Brienne carrying him from the baths, he'd never told her about his panicked ride back to Harrenhal after learning that she was in immediate danger, nor his subsequent leap into a bear pit with one hand and no weapon in order to save her; he certainly didn't tell her that he'd lost his hand defending the other woman in the first place. Though he supposed that Cersei's new pet Qyburn could have told her anything - he had been with them from their arrival at Harrenhal all the way to the capital. By now Cersei might've known about everything from the way Lord Bolton had taunted him with news of her after Stannis' attack, to the way he and Brienne had huddled together for warmth and helped care for each other's wounds on the road back to King's Landing. Whatever Cersei had said to Brienne that day had shaken her and she had stalked off to pace the outskirts of the affair.
Will leave you in the morning
As his troops continued the march north Jaime called to mind that image of the two of them together - the woman he'd loved his entire life, and the woman who had changed his life - and he remembered being afraid for Brienne. Cersei had looked like a predator toying with her prey - something he might have appreciated had her target been anyone else. When she later started screaming for heads on pikes in the aftermath of Joffrey's death, Jaime's first thought had not been of his brother (whom he'd assumed would get a fair trial if his father didn't put a stop to it before then), but of Brienne. Cersei could not be controlled, which meant lives were in danger. And once she had insulted Brienne to his face and demanded he break his oath to the late Catelyn Stark, he knew it wouldn't be long before she did something irreversible.
Cersei had forced his hand. He would betray his sister and conspire with the Maid of Tarth in order to preserve that oath. He'd had his trusted steward go to the armory and secretly commission new armor for her, to be delivered to him in the White Sword tower where Cersei and her spies never ventured. After finally visiting Tyrion in the dungeons himself, Jaime had then plotted with Bronn to prepare Podrick Payne to squire her, saving two birds with one net as it were. He'd planned on giving her one of the many swords kept in the tower when he unveiled her the armor, but once she was in Keep he realized that none of those swords were worthy of her, save one. He'd picked up the Valyrian steel sword given to him by his father. It had a sister sword that had been gifted to Joffrey on the fateful morning of his wedding. In true Joffrey fashion, he had named it something distasteful and had only brandished it long enough to destroy a priceless book.
He'd handed Brienne his sword as if showing it off before pronouncing that it was now hers. Those blue eyes of hers had gone wide and she'd tried to deny the gift but he'd told her of its origin - how the steel had come from Ned Stark's sword - and she finally accepted, vowing to find Sansa as a promise, now to him. He remembered being taken aback by that look in her eye as she held the sword - he'd seen it before somewhere on the road south with her - that look of imminent tears of unknown origin that never come. He'd helped her suit up, helping as much as he could with tying pieces into place with his hand before escorting her out of the Keep and into the nearby woods where Pod and Bronn had been waiting along with horses and provisions.
But find you in the day
Oathkeeper. The name stung at him as its sister pressed his hip during the long march through the rain. Of course she would name that sword something that would torture him.
He'd known that sending her away was right - even if Sansa was dead already, which he acknowledged had been a very reasonable assumption at that point, Brienne couldn't stay. It was too dangerous and he'd spent enough time protecting her, he wasn't about to allow the threat to come from his own door. But watching her leave, believing he'd never see her again - a woman who had spun his life around and made him see the deceit and corruption that he'd been blind to for so long - it was too much. He'd seen his eldest son die violently not long before, but that was a mere shade of the loss and worry he felt watching her ride away; without thinking he had started to follow her through the woods on foot. She'd looked back just once, catching his eye long enough to prove that they were both feeling the same loss. If his steward hadn't caught a glance from Bronn and acted as quickly as he had, Jaime might still be walking the path behind her horse to this day, wherever she was.
Instead he found himself on the road back to Riverrun, where he and Brienne had first met.
A/N: I do not own Game of Throne or these characters; some dialogue may be taken verbatim from HBO's Game of Thrones or George R. R. Martin's A Song of Ice and Fire. Lyrics used are directly from Andrew Belle's "In My Veins" (C) 2010.