This book is simply a travesty. I have to give Regan credit in his crafting of the narrative, however. He writes extremely well, but this is also one of the book's failings. In order to reach the level of engagement of the historical novel that this book does at times, he is forced to simply invent scenarios and the feelings of characters which go far beyond what we can do with the available source material. For example, Regan notes that Herakleios personally wanted to kill Phokas when the latter was brought to him during the usurpation, and that through this he was forced to grit his teeth (p. 55). The source material does not have this level of detail here, and while such things are fine for an historical novel, that is not what Regan set out to write.
This work is not purely descriptive. Regan clearly set out to argue that the Byzantines did have a concept of holy war, a topic contested in the scholarship. This is evident from his large quotation of Tia Kolbaba's work at the beginning of the book, as she wrote an important article on the topic. Dr. Kolbaba can rest assured that her thesis still stands in the light of Regan's book, because not only is his attempt to disprove it rather feeble, there are so many basic factual accuracies in this book that it even makes bad popular history. On p. 73, he uses the terms 'themata' and 'tagmata' with Herakleios. This is simply wrong. Although our first use of the term 'themata' appears in an entry by Theophanes Confessor in Herakleios' reign, this very may well be the use of later terminology for a period where it does not belong. The tagmata, on the other hand, were created by Constantine V in the eighth century. Regan claims that the Sassanids abutted the Chinese in the east (p. 41); this is not the case, and there is a long way between China and Sassanid Iran at all points in Sassanid Persian history. He also claims that Herakleios brought about the fall of the Sassanid dynasty (p. 126). This is not the case, as they would be overthrown by the Arabs in the decade and a half after Herakleios' victory. When he discusses the Arab conquest of Egypt, Regan uses extremely old scholarship. When bringing up Herakleios' new silver hexagram and the inscription on it (p. 71) he completely mistranslates it. On p. 195 he places Justinian's general Belisarius in the fifth century and not the sixth. Belisarius may have been born towards the end of the fifth century, but his entire career was firmly located in the sixth. On p. 33-4, Regan argues that the death and resurrection of Jesus only became fundamental part of early Christianity in the fourth century, which strongly suggests that he has never read either the Gospels nor any early Christian works. These constant mistakes serve to seriously undermine the integrity of this book.
When it comes to attempting to prove his thesis, Regan continues to falter by twisting and ignoring evidence to support his claims. He tries extremely hard to demonstrate that the wars of Herakleios are a holy war, but the support for this is so feeble that it falters. He argues that in 622 Herakleios was forced to turn to God and this led to the outbreak of holy war. Regan criticizes modern scholars for refusing to call a crusade a crusade. He claims that they are unable to see the forest for the trees, but it seems that Regan is unaware of the fact that the sources he's using are mostly late, and all are extremely tainted by the propaganda of the regime. As Regan himself notes later for his section John I Tzimiskes, propaganda does not make a holy war. Admittedly, Walter Kaegi's Heraclius, Emperor of Byzantium was not available when Regan was writing this book, but even a reading of the sources suggests Herakleios' constant desire to make peace. If this was a holy war bent on the destruction of the enemy, why was Herakleios so eager to end it, even with an unfavourable peace?
I could go on, but that would lengthen an already lengthy review. This is one of the worst pieces of popular history that I have ever read. It is full of inaccuracies, questionable writing for such a work, and it flounders when attempting to support its thesis. First Crusader is a terrible book that cannot be recommended.