It is a peculiar apple, the daughter of Kul Tiras's former Lord Admiral. Jaina's path in politics started only after she left Arthas at the gates of Stratholme. One might even say that her inability to persuade Arthas otherwise, drove him to madness at the end. The demise of Kael'thas and her father Daelin Proudmoore could also be attributed to Jaina. Yet we shall stay away from Jaina's influence on the men in her life. At least for now.
Her political career started at the gates of Stratholme. She broke off from Arthas, and like Thrall listened to Medivh's desperate call to flee and set sail for Kalimdor. Unlike all her predecessors she was different. Maybe it was her youth and inexperience. Maybe it was her compassion. Or maybe it was pure belief in the good in everyone. Instead of war, she was prepared to have peace. Even with Orcs. Even with Night Elves, who attacked and killed her men for chopping wood in Ashenvale. Sounds familiar? She saw benefits in working together with other races on this new continent, even though they all had reasons to hate each other.
After the war ended, and Archimonde was defeated by the wisps and coalition between the Horde, Night Elves, and what Alliance forces Jaina led to Kalimdor, they all went separate ways. Jaina founded Theramore, and established peace with the Horde. Thrall did not wish for war, Jaina wanted peace and everyone was happy. Except for a few warmongering Orcs that Thrall straightened with his Doomhammer.
Yet good things never last for long. Soon, Jaina's father landed on shores of Durotar. And unlike Jaina, he had pretty clear idea how to treat Orcs and their allies. Yet Jaina had a distinct idea of peace, and later on maybe even of prosperity in mind. She was willing to sacrifice all her father's forces, and her father as well, to ensure that idea of peace. Jaina let Thrall and his army invade Theramore and deal with her father. She betrayed her family to see this peace with the Horde stay in place.
Years went by and Jaina sat in her tower in Theramore. Her city grew. It served as the only Alliance outpost on eastern shores of Kalimdor. She still went on with her neutrality mantra. In Northrend we saw her keeping Varian on leash. When Varian wanted to start a fight in Undercity, it was Jaina who was disappointed and believed Thrall had a reason to discuss things with Sylvanas. It was Jaina who teleported Alliance forces from Undercity. It was also Jaina who preached peace to hot-headed Varian who refused to see past the impression that got engrained in his soul. And Garrosh only knew too well how to push Varian's buttons.
After the Ulduar fiasco, trying to bring Horde and Alliance together, Jaina tried to reason with Arthas in Halls of Reflection. Unlike Sylvanas, it seems that meeting Arthas as the Lich King broke her. Something deep inside her snapped and she left that place with her soul scarred even though she never touched the cursed blade Frostmourne. How would she not? Her first real encounter with death came at the hands of her first love. A man she probably still had feelings for.