Character Analysis
Phil "The Sultan of 'Stache" Stoneman is the eldest Stone-Bro and the only one of the bunch who survives the war. And he takes an unusual life path as far as Union veterans go.
That's because he falls in love with Margaret Cameron, Southern belle extraordinaire. Although she digs him, she's too scarred by her own brothers' deaths in the war to fully dive into their romance. He's a swell guy, true, but he's still a former Union soldier—the same dudes who killed multiple members of her family.
But Phil goes through some changes. After witnessing the reality (and by that we mean the film's reality—not actual historical reality) of Reconstruction, especially the death of young Flora, he starts taking the South's side. This culminates in his decision to shoot and kill a soldier who's trying to arrest Dr. Cameron, as well as protecting the family from a military unit that surrounds them after they hole up in a cabin.
In other words, this Yankee has become an honorary Southerner. From our vantage point in the 21st Century this shift might make us roll our eyes until they physically hurt, but it's this change that compels Margaret to marry the most magically mustachioed man who ever fought in the Union army.