Unite the Union named Unlikely Warriors as one of two "books of the month" for June 2013 and published a review by the historian Lewis Mates. Mates identified Baxell's inclusion of a chapter on those who fought for the Nationalists and "the wealth of detail [he] provides of the individual experiences of British fighters" as strengths, and praised his collation of published and unpublished accounts, interviews, correspondence and archival sources. Mates also published a review in the academic journal Contemporary British History in which he also noted Baxell's attentiveness to the conflicts between the International Brigades and the anti-Stalinist POUM, who some Communists claimed were acting on behalf of the fascists. However, Mates also described the six chapters dealing with events before and after the war as "a little disappointing", and questioned Baxell's treatment of Irish volunteers in Spain, his engagement with historiographical debates including whether the events constituted a war or a revolution and the International Brigades' relationship with the Communist International, and methodological questions relating to Baxell's varied sources. Concluding, Mates noted "that Unlikely Warriors is tailored to appeal to a wider market than simply academics" and described the book as "a well researched, largely balanced, highly readable and accessible narrative".
