There is some dispute about whether this new military structure was put into place under the Emperor Diocletian or Constantine since both reorganised the Roman Army in the late 3rd and early 4th centuries to some degree. Both Diocletian and even his predecessor of thirty years Gallienus may already have controlled mobile strategic reserves to assist the empire's border forces;* Southern & Dixon, "The Late Roman Army", pp. 11–17</ref> either Diocletian or Constantine expanded this nascent force into permanent field armies.