Pacifism was rare in the Middle Ages; the vast majority of theologians, following in the footsteps of Augustine, considered that there were valid reasons for Christians to go to war, so long, that is, as this war were just. To that end, they developed a theory of just war based on three principles; in order to be just, a war must meet the conditions of just cause, proper authority, and correct intention. The author's argument is that John Wyclif (c. 1330–1384) rejected each of these conditions, effectively undermining just war theory and making Wyclif the first medieval pacifist. In this, the author goes against the modern scholarly consensus, which has pictured Wyclif as either moderately approving of, or rather indifferent to, just war theory.

After outlining the development of just war theory through to the fourteenth century, the author discusses Wyclif's rejection of each of the principles of just war theory in three successive chapters (2–4), which form the heart of this study. Just causes for war in medieval theories were causes such as self-defense, material causes (protection of goods and so forth), right of conquest, or punitive causes (vengeance for wrongs committed by the other party). The author demonstrates how Wyclif rejected all of these based on his reading of scripture, especially the Sermon on the Mount (Matthew 5–7), which was a message of love and self-sacrifice that formed the basis of Wyclif's understanding of what he called the lex Christi (the law of Christ, a term interchangeable with lex caritatis, the law of love, or lex ewangelica, the gospel law). Because the Christian life is based on love and self-sacrifice, killing, even in self-defense, becomes highly problematic, because it is less evil to die than it is to kill. This is especially so for Wyclif, since he rejected the idea of double effect; he demonstrates a profound skepticism that an evil act such as killing can have a positive effect. As to proper authority, there was some debate among theologians as to whether only emperors had the proper authority to wage war justly, since they alone had universal authority, but it was generally conceded that kings, too, had this authority. This was undermined by Wyclif's theory of grace-founded dominium; only someone in a state of grace could legitimately exercise lordship of any kind in Wyclif's view. Effectively this meant that no ruler could be sure to have the necessary authority to pursue just war, as such authority is God's alone and special revelation would be necessary to determine if a ruler was in a state of grace which allowed him proper authority to pursue war. Wyclif's insistence on the primacy of love (caritas), and the opposition of this love to violence, meant that for Wyclif justice, which must be based in love, and war were mutually exclusive, undoing the possibility for combatants to enter into a war or commit a killing with correct intention. These three chapters form a coherent and convincing argument for Wyclif's theological dismantling of medieval just war theory, drawing from the entire breadth of Wyclif's corpus to demonstrate the author's thesis.

In the final two chapters of the book, the author traces the importance of this rejection of just war for Wyclif's political theology, and outlines his moral rejection of war. In the first of these chapters, Wyclif's utopian ideal of an evangelical society ruled by the lex Christi is explained. The argument is that Wyclif believed this state was attainable temporally, and not just in the eschaton, through an inevitable regenerative process of reformation that would return the church to its primitive, pristine condition. Here I find the argument less convincing, as Wyclif seems to me, while enthusiastic for reform in doctrine and morals, to be skeptical that true regeneration can be achieved before the Day of Judgement, as his numerous references to the Fatherland (i.e., heaven) and the conditions of life there indicate. The final chapter is more speculative than the rest of the book, exploring, among other things, the possibilities for the intersection of Wyclif's realist metaphysics and his espousal of pacifism, and ending with a coda discussing the influence of Wyclif's thought on Lollard and Hussite pacifism, and a call to examine further this last connection.

There are a few minor issues with the book, which were, one would think, editorial decisions. References to Wyclif's Latin works include only page numbers; it would have been more useful to reference line numbers in addition. As well, it would have been convenient to the serious reader if the original Latin for quotations were included in all corresponding footnotes, instead of only selected passages. But these are minor quibbles. Overall, this is an original and engaging piece of scholarship, which will no doubt lead other Wyclif scholars to readjust their understanding of Wyclif's views on war, peace, and politics. This is a book that demonstrates the positive effects that engaging old material with fresh questions can have, and the author is to be commended for his contribution to Wyclif studies.