He absolutely could not tell her that he loved her–at least not until the a treaty was signed. Because loving her meant understanding that this war came first, and putting his own feelings aside for now. Because no matter how she responded, it was going to put them both at risk.
Either, she loved him too, and his confession took her mind at least a little off the strategy to imagine what their life could be like in the future. Then if anything went wrong, she would quietly blame herself and him for the distraction, no matter how slight, from allowing her to be the best she could be.
Or, she didn’t feel that way at all. And then she was properly angry at him for introducing any kind of distraction when both she and him should be focused on the war they were fighting. She would be mad that he was thinking about her when he should be working, and she would be mad that now she was dedicating time to being mad at him instead of doing her work, and the part of her that was his friend would be worrying that she would be spending too much time pining over her, and getting himself killed in the process.
Or the most likely option of all, She wouldn’t even have considered it. She was raised in a rebel family during a war for most of her life–both her parents were fighters, her brother was a fighter, and she’d seen her first battle at eleven years old. When you’re life is a battle for survival, it’s not like you’re spending a lot of your time focusing on romantic interests. And she wouldn’t have time to consider it now. Why would he ruin his chances for her to consider him in a positive light by making her think about it now?
No, he couldn’t admit he loved her. Not until the treaties were signed. And there was nothing quite as good a motivator for him to make absolutely sure that peace was going to stand.