In 1636: The Cardinal Virtues, Eric Flint and Walter Hunt move the action west into France where the ongoing strife between King Louis and his loyal minister Richelieu and his brother the scheming Monsieur Gaston, Duc d'Orleans is boiling over. Richelieu has concocted a plan to get the barren Queen Anne pregnant and to provide Louis with an heir, a plan that would see an end to Gaston's long-held dream of supplanting his brother on the throne. Upon learning of this pregnancy, Gaston chooses a desperate course, orchestrating the assassination of Louis and Richelieu and allying with Spain to put himself on the throne. All that stands in Gaston's way are Richelieu's picked men, a secret society of noblemen sworn to defend the crown, Queen Anne, and a handful of up-timers.
This book picks up directly after the events depicted in Eric Flint's story in the last Grantville Gazette, in which the USE and France made peace (as Richelieu finally realized that he couldn't prevent the influence of the Ring of Fire from spreading and it was best to accept it and move on). This is important to note because the book starts off with France and the USE on fairly friendly terms with everyone talking about how they aren't enemies anymore but if you've just been reading the main books then the last time the USE and France had anything to do with each other was 1634: The Baltic War, and that was less than collegial.
I've long thought that France was a weakness in the Ring of Fire stories. Flint's stories have kind of glossed over the Huguenot issue (considering how the Irish rebels have gotten such repeated inclusion, it seems rather unfair given how badly they got screwed in our history that their only presence is in the form of a band of rather unhinged assassins and a handful of character playing "don't ask, don't tell" on the issue), Richelieu got painted as a major Opponent early on and then Flint decided that was unfair to the historical character and hid him in the background long enough to make it reasonable for everybody to make peace, and the French in general often are thrown into stories as enemies of convenience (see 1636: Commander Cantrell in the West Indies, for instance). There are repeated references to the clash between King Louis and Monsieur Gaston in previous books, but frankly, until now it was such an obscure element (and of limited importance to the main story) that I can't say it really interested me. Well, this book rectifies quite a bit of that. The French may not be the most likeable characters in post-Ring of Fire Europe, but this book manages to at least flesh out French politics of the time enough to finally provide some depth to characters such as Richelieu, Gaston, Louis, Mazarin, and Turenne.
As a side-novel in the Ring of Fire series, Cardinal Virtues is both a success and a failure. On the success side, the novel is interesting and well-written with several interesting new characters and a fluid unpredictable narrative. On the failure side, Cardinal Virtues is the fourth RoF side-novel in a row that lacks any sort of conclusion (basically, every RoF novel that has 1636 in the title apart from 1636: The Devil's Opera). Frankly, if you were hoping for a story of French armies clashing in Paris, then this is not the novel you wanted. As other reviewers have pointed out, this book details the events leading UP to the French Civil War, but apart from various back-alley intrigues, all the fighting is between the French and the Spanish (and it's just one chapter). So Flint and Hunt get all the pieces in place with everybody picking sides (could the Netherlands FINALLY tell the Spanish to go to hell? It's about freaking time!) and the story ends. The story is also a bit of a failure from its place in the series as a whole. If you were hoping the story would touch on ongoing elements like the role of the Committees of Correspondence in France or give some hints as to what happened in Italy after the Mallorca incident, then tough luck, because the CoC isn't even mentioned (seems insane with France on the verge of a nobility-driven civil war that the peasantry isn't getting disgruntled) and events in Italy are left pretty much where they were in 1635: Papal Stakes with the two popes locked in an apparent Holy Cold War. Apart from the work done to flesh out France, the only really interesting bits (from a world-building perspective) is that the book has a chapter detailing how the USE's government is functioning since the end of its pseudo-civil war.
All in all, Cardinal Virtues is a well-written book and it's interesting but it's incomplete and we've now got at least five hanging threads in the Ring of Fire universe from incomplete novels: the West Indies storyline, the Austrian storyline, the Russian storyline, the main storyline, and now France. I'm hopeful that the next book, which picks up the story in Britain, will be self-contained, but I'm probably being overly optimistic (UPDATE (1/20/2016): I was, make it 6 hanging threads). All of which seem to be waiting on the bloody Turks to launch their invasion of Austria just so we can get the story moving forward again, and from what Eric Flint says, we're not getting that book until 2017! GAH! I love this series, but it's starting to feel like it's getting bloated on too many side stories and not enough plot/world progression.