This book has been long in the making. It began as a project to understand the development of the Roman army and its cavalry forces of which the reigns of Aurelian and Probus formed only a small part. The first manuscript was ready in 2008 after which it was shelved until 2016. It is thanks to the encouragement of my wife and children that the book was taken out of the old files and finishing touches added. However, new illustrations were added later in 2018 and 2019 to replace some of the older ones.