I think that idealism and reasons of war both have places in the novel.
The elderly peasant Anselmo most fully represents the Loyalist ideals in the novel. Hemingway suggests that his lack of education and his compassionate nature allow him to believe in the cause and to fight for it to the end of his life. Through his idealism, he supplies the human element to the struggle that Jordan and Pablo so often ignore.
Pablo has largely forgotten the ideals of the cause to which he had originally devoted his life. He has seen too much of the reality of war and so participates now more out of self-interest than out of patriotism. As a result, he can take pleasure in his brutal murder of the Fascists. And when he considers the plan to blow up the bridge too dangerous, he flees with the explosives. Yet he appears to retain some of the ideals to which he once dedicated himself. When Pilar asks him why he did not kill Jordan when he had the opportunity, Pablo replies that Jordan is "a good boy," since his motives are noble. He also notes the camaraderie that results from devotion to the cause when, as he describes his desertion, he notes, "having done such a thing, there is a loneliness that cannot be borne."