The actual source is this video where he spends almost two minutes trying to say how the conflict itself was bad and thinking if it was negotiated there wouldn’t have been a war.
Don’t know the full video, but I would like to believe he was trying to show sympathy for the actual losses and casualties. It’s Trump so he was probably going on a tirade on how if he had negotiated that there wouldn’t have been a need for a civil war, or something along those lines.