"The Underdogs" is in no way a historical novel (characters are purely fictional), nor journalism (events are imagined), nor a non-fiction novel (Azuela would have stared at you for so calssifying his novel). It is a realistic novel, fictional but firmly rooted in social reality. Azuela was a doctor in the armies of Pancho Villa, and so he directly witnessed action in the field. He was an upper-class, educated man who joined the Revolution as a medical expert. His novel, while fictional, tells the truth: that most revolutionaries joined in purely for personal, local reasons; that they knew little or nothing about the larger political picture; that they were lost in the maelstrom of something that, more than a "classical" revolution, was simply the total, sudden collapse of a system of archaic institutions that had been freezed by a decrepit dictatorship. When the really fictional system fell, all hell broke loose and everybody took up arms to exact revenge from whoever they blamed for their poverty and misery.
Villa was the quintessential revolutionary man: a very poor peasant, whose family was oppressed by the feudal lord who owned the land where they lived and toiled, he became a mountain bandit, a cattle-robber, an outlaw. He had charisma, wits, and big balls. He became a legend in his twenties. His light cavalry was a nightmare for the rigid military who hopelessly chased him through the most inhospitable terrain. He was also a psychopath: a man who would go from tender tears to murderous rage in seconds. His best biography, "The Life and Times of Pancho Villa", by Friedrich Katz, is epic and unbelievable. It also explains Mexico's wild North and the crucial part it played in the Revolution.
Azuela's revolutionaries are indeed mestizos, men alienated from the primitive indigenous communities, but also cut off from urban society. Poor peasants with a patch of dry land, subject to any and all arbitrariness by landlords straight from the European X Century. They have literally nothing to lose. They're out for blood and money. They are doomed to failure because, in the end, urbanites would win the war and install the regime which will, most likely, be dumped into history's garbage can come next July 1st.
I see two ways that I misspoke (mistyped?) in the earlier post. D'Artagnan was indeed a real person, but an obscure one. Dumas fabricated many adventures for him. D'Artagnan interacts with important historical figures such as Cardinal Richelieu, but no one would mistake the Musketeers novels for history or nonfiction.
By my own definition, The Underdogs would not be a roman Ó clef because the characters are all obscure. I'm not certain where you would classify straight journalism in which the names are all changed. I imagine most writers would explain that it is true, but that "the names have been changed to protect the innocent." It would be a rare case indeed in which journalism would masquerade as a novel. I have no reason to believe that Azuela did not invent his characters, even if he created them from bits and pieces of people that he had known. Don't novelists do this all the time? Guillermo, is there any reason to believe that the characters and events in The Underdogs are literally nonfiction?
It is interesting to me that you believe The Underdogs to be closer to reality than Herzog , when the latter is explicitly autobiographical. Indeed, the real person on whom Valentine Gersbach is based wrote an "answer" novel that presumably presents his side of the Herzog story. So the Bellow novel may be much closer to reality than The Underdogs .