Finally Steve understood.
Understood what drove Tony to try to build Ultron.
Understood why he wanted them to sign the Accords.
He understood why Tony wanted the Earth to be united.
Because Tony had known this was coming. He had known that the Earth would someday face this kind of threat.
The words Thor had said so long ago came back to Steve. "It is a signal to the Realms that Earth is ready for a higher form of War!"
Steve thought he had known war. But he had forgotten that Tony's life had revolved around countless wars, all of his life. His mind was always thinking of ways to defeat his opponent. And Steve wonders, if Tony was here, would things have turned out differently?
Staring at the ground where Bucky had disappeared in a cloud of dust, he finally understands and regrets.