The Greatest Story in the World, Period 2 by Horace G. Hutchinson

The causes that led to the dividing up of the Empire are easily / understood. What is far less easy to understand is how Rome ruled / the world, as the world then was known, so long as she did. Remember / this: at that time you could only travel, and you could only send a / message, as fast as a horse could gallop, if it was by land that you / went or sent; and only as fast as a ship -- a ship with a very simple / and primitive way of setting the sails -- could be urged through the / water by sailing or by rowing, if your going was by sea. For / practical purposes of getting news or of moving troops, the world of / the Romans of that date, say from Egypt to Britain at its furthest / points, was a very great deal larger than the whole of the globe is / to us to-day. If you can understand it in that sense, their Empire / was very much larger, much less under the eye and the direction of / the centre of government, than the whole British Empire to-day. And / we find that large enough. The Romans had the further trouble, which / we have not, that the leaders of the legions in the provinces, when / they had repelled the barbarians, sometimes claimed to be independent / of the central authority, as we saw both in Gaul and in Asia Minor.

So the wonder really is, not that Rome should at length fail to / govern all this Empire from one centre, but that she should have / succeeded in doing so at all, {24} and for so long. From causes / which I have spoken of already, the home government was not as strong / as it had been; and as the power at the centre grew less the pressure / of the barbarians on the boundaries grew more. Especially it became / convenient to have a centre of government nearer the boundary on the / south-east, where the eastern barbarians were constantly making their / attacks and where a great leader of the army, if he checked the / attack, might become too strong for the authority of Rome to control / unless it put forth all its force. A solution of the trouble was / attempted by the Emperor Diocletian, who came to the imperial throne / in 284. What he did was to appoint a colleague for himself to whom / he gave his own title of Augustus, though he also retained the title / for himself. There were, therefore, two Augusti. And besides the / Augusti, he appointed two leaders of armies in the provinces to bear / the title of Cesar. Thus there were two Cesars and two Augusti. The / Empire and its armies were portioned out between these four great / persons. Diocletian himself had the command of the army of Syria. / His colleague, the other Augustus, commanded the armies of Italy and / of Africa. One of the Cesars had the armies guarding the Rhine, and / the other the armies guarding the Danube boundary.