The idea of ​​Roman grand strategy has been an often debated topic. Edward Luttwak initially put forward the idea that during the crises of the third century, Roman grand strategy began to move toward a defense-in-depth approach, removing the boundaries of their defenses and creating a large, mobile field army. Thereafter, the defense-in-depth approach remained the major Roman strategy used throughout the third to fifth centuries. Arther Ferrill also corroborates this account of a shift in Roman grand strategy toward a defense-in-depth approach. He cites Constantine as the emperor who created a mobile field army that constantly stayed with the emperor and suppressed border defenses. Subsequently, several mobile field armies became necessary to actually defend the empire. He argues that this change in strategy contributed greatly to the ultimate destruction of the Roman Empire, because defense in depth essentially amounted to admitting that it was impossible to prevent a foreign invasion. Meanwhile, many other ancient scholars have argued against the idea that the Romans employed a defense-in-depth method. traditional grand strategy at all. Benjamin Isaac objects to the assumption that Roman military strategy operated according to a logical and well-thought-out plan. The idea of ​​grand strategy assumes that the emperors' decisions to engage in battle were necessarily rational. It often seems that emperors act impulsively, simply trying to avoid internal upheaval. This impulsiveness goes against the idea of ​​a prescribed long-term strategy. To discuss the idea of ​​a grand strategy, one must first acknowledge the inevitable difficulties of finding and interpreting evidence for or against such a strategy. It's difficult to talk about a Roman... middle of paper ... defining Roman grand strategy. " The Journal of Military History 70.2 (2006): 333-63. JSTOR. Web. April 1. 2010. Libanius and AF Norman. Autobiography and selected letters. Cambridge, Mass.: Harvard UP, 1992. Print. Autobiography and selected letters Cambridge, Mass.: Harvard UP, 1992. Print. Luttwak, Edward. Strategy of the Roman Empire: From the First Century to the Third. Baltimore: Johns Hopkins UP, 1979. Print. life of the sophists Cambridge, Massachusetts, 1989. Print.Zonaras, Joannes, Thomas Banchich and Eugene Lane The History of Zonaras: from Alexander Severus to the Death of Theodosius the Greater London: Routledge, 2009. Print.Zosimus and Ronald T. Ridley. History Sydney: Australian Association for Byzantine Studies, 2006. Print.