There had indeed been sundry attempts to annex still more of the world. Roman armies had crossed the Rhine and had twice fought their way to the Elbe; but it became apparent to the shrewd Augustus and Tiberius that the country could not be held, and the Rhine was for the present accepted as the most natural and practical frontier. In the East the attempts permanently to annex Armenia, or a portion of Parthia, had so far proved but nominal or almost entirely vain. 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.