This is the theme one would expect to find in every Hemingway book, and For Whom the Bell Tolls doesn't disappoint. "Being a man" is an ideal of many of the characters, one men hold up for themselves and one women criticize them for if they fail to meet. The war places particularly high demands on "manliness." Courage, and willingness to risk one's life, is one standard used to separate the men from the boys (or women). But perhaps still more important – and central to the protagonist – is the ability to control one's emotions and urges, and to "take it straight" (just accept reality and deal with it – no whining). The protagonist might be seen as an ideal male by those standards.
Robert Jordan subscribes to a vision of "tough-guy" masculinity to which he himself fails to live up.