 Both Diocletian and even his predecessor of thirty years Gallienus may already have controlled mobile strategic reserves to assist the empire's border forces; either Diocletian or Constantine expanded this nascent force into permanent field armies.Recruitment from amongst Roman citizens had become greatly curtailed as a consequence of a declining population, "cripplingly numerous" categories of those exempted from military service and the spread of Christianity with its pacifist message.Together, these factors culminated in "the withdrawal of the urban class from all forms of military activity."