The usurpation of power in Castile by the Trastamáran dynasty in 1369 ushered in a period of violent turbulence and civil war. Under Isabella the Catholic and Fernando of Aragon, Grenada—the last Muslim kingdom in the Hispanic Peninsula—was captured in 1492, and the foundations laid for Spain's Golden Age. Claussen convincingly argues that this glory was in part achieved by harnessing the very forces and ideology that had created the Trastamáran chaos.

The murder of King Peter I (1350–69) unleashed violence and war within Castile that was justified and driven by the cult of chivalry pushed to extremes. The author follows Kaeuper in seeing chivalry as a self-interested and doctrinaire assertion of the autonomy of the noble warrior and his absolute right to protect his honor and that of his lineage in all circumstances. For Claussen, it is not a civilizing force, as some—notably Jesús Rodríguez-Velasco (Order and Chivalry: Knighthood and Citizenship in Late Medieval Castile [2010])—have argued, let alone the gentrified morality of Victorian legend. The Trastamáran kings, having usurped the throne by violence, were in debt to their supporters, who did not hesitate to follow their example and justify violence as chivalry, even when this amounted to civil war and resistance to the Crown.

This was a literate aristocracy, consciously appealing to chivalry embodied in a remarkable literature in Spanish. The central thrust of Claussen's work is to explain the violent behavior of the Castilian armsbearers in terms of the ideology embodied in this literature, which is here subjected to a striking and forceful examination. Neither the cult of chivalry nor the collision between a centralizing monarchy and an aristocracy set on defending its autonomy was distinctively Castilian. But in the Trastamáran period this reached a particular intensity, and, as Claussen shows, created a sharp reflection and justification in works of chivalry. What was at stake in this debate was the nature of monarchy. Loyalty was a chivalric virtue, but nobles claimed to be loyal even when fighting against the king. Claussen argues (69–103) that in this ideology townspeople and peasants were little more than useful animals, subjected with no right to self-defense or noble violence. The Crown was perfectly aware of the cost of noble turbulence, but did little about it.

It seems odd at first that, despite assertions of independence, there was no drift away from the Crown and its government. Claussen shows very clearly that there was a strong current of loyalty in Castilian chivalry, and this acted as a binding force. But there were surely other factors at work. The great lords, in effect, saw government as a joint enterprise with the Crown to exploit the rest of society. The link between Crown and nobility was surely more than simply ideological.

However, Claussen is quite right to assert that there was a counter-prevailing force—for chivalry praised loyalty and honored holy war. All over Europe the crusade was a vital part of chivalry. But, as Claussen rightly emphasizes, this was especially the case in Castile, which was in constant contact with Islam. The call to holy war was a unifying force that emphasized the supremacy of the Crown, and, as Claussen demonstrates, Trastamáran rulers who practiced it did better than those who did not. But, of course, rulers had many other priorities, and since Islam was hardly a threatening force, holy war often had a low priority. But holy war was certainly the key to the success of Isabella the Catholic, who was a tireless advocate and leader in this enterprise—reversing, as Claussen shows, the passive role that contemporary chivalry assigned to her gender. By this means she harnessed chivalric violence to the cause of the monarchy.

This is an important book in that it demonstrates how literature and political reality interacted, exposing the reality of Castilian chivalry, and that, while it shared much in common with other parts of Europe, there was a distinct, intense, and perhaps malign flowering in this period that has left its mark on Spanish politics to the present day.