On the Upper Euphrates at this date there was a sort of acknowledgment
of vague dependence on Rome, but the empire had acquired nothing more
solid. Forty years before our date a Roman expedition had penetrated
into South-west Arabia, of which the wealth was extravagantly
over-estimated, but it had met with complete failure. Into Ethiopia a
punitive campaign had been made against Queen Candace, and a loose
suzerainty was claimed over her kingdom, but the Roman frontier still
stopped short at Elephantine. Over the territories of the semi-Greek
semi-Scythian settlements to the north of the Black Sea Rome exercised
a protectorate, which was for obvious reasons not unwelcome to those
concerned. Along or near the eastern frontier she well understood the
policy of the "buffer state," and, within her own borders in those
parts, was ready to make tools of petty kings, whose own ambitions
would both assist her against external foes and relieve her of
administrative trouble.

At no time did the Roman Empire possess so natural or scientific a
frontier as at this, when it was bounded by the Rhine, the Danube, the
Black Sea, the Euphrates, the Desert, and the Atlantic. The only
exception, it will be perceived, was in Britain, but the Roman idea
there also was to annex the whole island, a feat which was never
accomplished. Two generations after our chosen date Rome had conquered
as far as the Firths of Clyde and Forth; it had crossed the Southern
Rhine, and annexed the south-west corner of Germany, approximately
from Cologne to Ratisbon; it had passed the Danube, and secured and
settled Dacia, which is roughly the modern Roumania; and it had pushed
its power somewhat further into the East. But it had not thereby
increased either its strength or its stability.