I've got a weirdly specific recommendation here, but hear me out. It's not a pure romance, but Bernard Cornwell's 'Sharpe' series. It's historical military fiction, but the romantic subplots with characters like Lady Grace Hale or Teresa Moreno are incredibly passionate and permanently shaped by the Napoleonic wars he's fighting through. The relationships are born in chaos, defined by long absences and the constant threat of death, and they feel painfully real because of it. There's a rawness to the emotions that a peacetime setting just can't replicate.
I'm less enthused by some of the dedicated Civil War romances that feel like they're using the war as a dramatic backdrop for a fairly standard 'forbidden love' plot between a nurse and a soldier. I want the conflict to be baked into the characters' bones, not just a scenic wallpaper. Another one that pulls this off is 'The Colonel and the Enchantress' by Paullett Golden, which is Georgian, not American Civil War, but gets that same mix of military duty versus personal passion.
For the American conflict specifically, 'The Blue and the Gray' series by various authors has its moments, though quality varies. I remember one where a Southern belle falls for a Union spy, and the tension wasn't just about their sides, but about the sheer moral exhaustion of the war. The passion came from shared despair as much as attraction, which felt more authentic to me than grand, sweeping declarations on a battlefield.