It was, looking back, obvious that this would happen. Arthur knew that he should have seen the signs. Alfred's rebellion was not a secret. He saw it coming and foolishly brushed it aside. He thought it was a faze. It wasn't. He thought it would pass quickly. It didn't. He thought he could win. He couldn't. One of his generals knew that. He actually brought the man home on his request. When the general told him that Alfred's determination was so strong that the man was living off sweet potatoes and dirty stream water, he didn't believe him.
He was an idiot.
He hurt not only Alfred, his little brother by bond, but he hurt the man's kids, the states. He had a few chances to take Alfred out. He stopped himself from firing a kill shot every time.
At the end of the war...
Alfred and Arthur were standing face to face, guns pointed at each other. Alfred stood tall and ready while Arthur wavered, unwilling to fire a kill shot at his younger brother. He closed his eyes and dropped his gun, falling to his knees.
"I can't!" Arthur yelled. "I can't shoot you..."
He heard Alfred step closer to him and say, "I no longer consider you my brother. Now, get off my land."
Arthur got onto his ship and headed home. Collapsing in his bed, he broke into tears from Broken Brotherly Bonds.
It takes a lot to fix them.