The History of the Roman Empire

The Persians, the Arabs, the Slavs, the Bulgars and the Germans were by now significant nations right on the border. The discipline of a professional army was still an advantage over enemy volunteers. As had happened at Adrianople, the loss of a whole army could have disastrous consequences - not to mention being hugely expensive. To destroy a whole people might only make their neighbours stronger. In a world with multiple enemies, the Byzantines also recognized that today's enemy could be tomorrow's ally.