Even in a military point of view the boundary of the Tigris was easier of defence than the previous frontier-line which ran along the Syrian desert and thence along the Euphrates. The conversion of the region of Adiabene beyond the Tigris into a Roman province, whereby Armenia became an inland one, and the transformation of the Parthian empire itself into a Roman vassal-state were corollaries of the same idea. It is not meant to be denied that in a policy of conquest consistency is a dangerous praise, and that Trajan after his fashion yielded in these enterprises more than was reasonable to the effort after external success, and went beyond the rational goal;[53] but wrong is done to him when his demeanour in the East is referred to blind lust of conquest. He did what Caesar would have done had he lived. His policy is but the other side of that of Nero’s statesmen, and the two are as opposite, as they are equally consistent and equally warranted. Posterity has justified more the policy of conquest than that of concession. [Sidenote: Reaction under Hadrian and Pius.] For the moment no doubt it was otherwise. The Oriental conquests of Trajan lit up the gloomy evening of the Roman empire like flashes of lightning in the darkness of the night; but, like these, they brought no new morning. His successor found himself compelled to choose between completing the unfinished work of subduing the Parthians or allowing it to drop. The extension of the frontier could not be carried out at all without a considerable increase of the army and of the budget; and the shifting of the centre of gravity to the East, thereby rendered inevitable, was a dubious strengthening of the empire. Hadrian and Pius therefore returned entirely into the paths of the earlier imperial period. Hadrian allowed the Roman vassal-king of Parthia, Parthamaspates, to drop, and portioned him off in another way. He evacuated Assyria and Mesopotamia, and voluntarily gave back these provinces to their earlier ruler. He sent to him as well his captive daughter; the permanent token of the victory won, the golden throne of Ctesiphon, even the pacific Pius refused to deliver up again to the Parthians. Hadrian as well as Pius earnestly endeavoured to live in peace and friendship with their neighbour, and at no time do the commercial relations between the Roman entrepôts on the Syrian east frontier and the mercantile towns on the Euphrates seem to have been more lively than at this epoch. [Sidenote: Armenia a vassal-state.]