Holy War is an important book connecting the history of Italian colonialism, the Vatican's relationship with Fascism, and the longer trajectory of contacts between the Catholic and the Ethiopian Orthodox Churches. It expands on the author's trilogy on Italian colonial violence in Ethiopia between 1935 and 1942: The Plot to Kill Graziani (2010), The Massacre of Debre Libanos (2014), and The Addis Ababa Massacre (2016). In this volume, Campbell offers an innovative and thought-provoking reading of the entanglement between religion and colonialism in Ethiopia under Italian occupation.

The argument of the book is that the destruction of churches and the persecution of the Ethiopian clergy were part of a modern crusade waged by the Italians against the Ethiopian Orthodox Church with the support of the Catholic Church. The book reframes the infamous massacre of the monastery of Debre Libanos from an exceptionally violent episode of colonial violence ordered by the viceroy Rodolfo Graziani to the tip of the iceberg in a much larger and systematic campaign of terror that has been largely undocumented or underestimated in the archival record. Campbell's most innovative and original contribution is to integrate Italian state sources with interviews with Ethiopian survivors and on-the-ground fieldwork on the sites of massacres, which are illustrated with several maps and photographic evidence from the time. This approach allows Campbell to reconstruct in detail the sobering account of widespread violence that the Italians perpetrated in several Ethiopian provinces, where local information about these massacres has been largely ignored by historians.

After a prelude summarising relations between the Catholic and the Ethiopian Orthodox Churches in the past, the author reconstructs the ‘unholy alliance’ between the Vatican and Fascist Italy ahead of the invasion of Ethiopia. Building on the work of Lucia Ceci, Emma Fattorini, and David Kertzer, Campbell explains how the Vatican shifted from friendliness to condemnation of the Ethiopian Orthodox Church as a schismatic institution in the months ahead of Italy's invasion of Ethiopia. Because of the Holy See's commitment to the alliance with Mussolini, Pope Pius XI adopted a policy of vacillation towards the invasion that was meant to reassure the international Catholic community while letting the Italian clergy stir excitement for a modern crusade in support of Italian imperialism. The beatification of Giustino de Jacobis, a Catholic bishop who had served in Ethiopia in the mid-nineteenth century, statements by influential Italian clergymen such as Cardinal Schuster, Archbishop of Milan, and the cultivation of the myths of the Madonna del Manganello and Father Andrea Giuliani strongly suggested that the Italian clergy saw in Mussolini's invasion of Ethiopia an opportunity to expand the Catholic faith against a branch of Christianity that was considered backward, heretical, and far too close to Jewish ritual practices. In short, the Catholic Church sided with the Fascist regime to promote popular support for a colonial crusade.

The core of the book examines the destruction of churches and monasteries during the 1935–6 campaign and the subsequent repression of Ethiopian patriots by Italian soldiers and their Muslim troops. Campbell points out that these attacks were often distant from the main military theatres, but churches were lucrative targets for pillage during the invasion. While Marshal Badoglio tried to limit the destruction of churches as a future hindrance to the pacification of Ethiopia, Graziani reversed this policy because he considered religious sites as ideal for exemplary reprisals against the Ethiopian resistance. A key example is Campbell's analysis of the Debre Libanos massacre, during which Graziani ordered the slaughter of 1,800 to 2,200 innocent people, including monks, visitors, pilgrims, and students who were at the site for a religious holiday and had no connection with the plot to kill the Italian viceroy. Campbell demonstrates that the number of victims was underreported in Graziani's telegrams to Rome, as he tried to cover up his actions by imprisoning about 360 residents of Debre Libanos and family members of the executed clergy in Somalia. Thus, Campbell brilliantly completes patchy Italian archival records with his research on the ground and convincingly demonstrates that the destruction of churches in the Addis Ababa and Debre Libanos massacres were only the most famous cases in a much broader strategy of colonial violence against the Ethiopian Orthodox Church.

Finally, the book examines the reversal of Italian religious policy in Ethiopia under viceroy Amedeo d'Aosta, who aimed at turning the Ethiopian Orthodox Church into a tool of Italian occupation. After Graziani, Italians switched tactic from destroying the Ethiopian Church to pausing Catholic proselytisation, appointing new Orthodox clergymen who were autonomous from the Egyptian Orthodox Church, and even rebuilding churches and monasteries. This change of strategy caused the disappointment of the Catholic Church and Italian missionaries already sent to Ethiopia.

Holy War opens up several new perspectives for future research. For example, Campbell asks how many Ethiopian religious artifacts are still in Italy illegally, an important topic at a time when former colonial powers negotiate the return of looted artifacts of artistic and cultural significance. Additionally, the book is bound to stir a debate about the use of the word ‘pogrom’ to describe Italy's crusade in Ethiopia. The author suggests that the collaboration between Italian clergy and the Fascist state degraded the image of Ethiopians and the Orthodox Church to the point that reluctant Italians were successfully mobilised to volunteer in the campaign in the name of their faith. The vast impunity of Italian soldiers confirmed that their violence served a right and just cause. Still, the question remains as to whether Italy's crusade against the Ethiopian Orthodox Church was a spontaneous religious persecution or a series of actions planned by the state with the support of higher Catholic hierarchies. In short, how pervasive and successful was the message of the crusade in Italian society? In conclusion, Holy War stands out as one of the most important recent contributions to the history of Italian colonialism, Fascism, and their relationship with the Catholic Church. The book is an invaluable testament that preserves the voices of Ethiopian survivors of Italian colonial violence in the historical record.