War and romance are the two central elements of the book. It's meant to be "about" war, and it's the situation of war which generates all of the central tensions and conflicts in the book. Then there's the love story between Robert Jordan and Maria, which is the central component in Robert Jordan's own personal transformation narrative.
The contrast and interplay between the two is essential. It's by being set against this promise of happiness in love that Robert Jordan's choice to risk his life instead and serve the military cause becomes agonizing, and heroic. Life's full of tough choices, isn't it? On the other hand, you might see the contrast between love and war another way: might it be the case that there really is no choice, since ultimately, so long as the war continues, the happiness of love would be impossible – hence the evil of war?