A history of the art of war under the Roman Empire has not yet been written, for the simple reason that we do not possess an account by a good military historian of a single campaign between that of Thapsus (46 B.C.) and that of Argentorate (357). Josephus does indeed give a first-hand account of the Jewish war of 66-70, and took some trouble over military details, but his subject limited him to siege operations and street-fighting. The most valuable section in his work is a general sketch of the Roman army and its organization, and a description of the arrangement of troops on the march.[311] Tacitus, on the other hand, who is forced by his subject to describe several campaigns, and remains in consequence our chief authority, cared nothing for the technical side of warfare, and does nothing more than record, as a rule correctly enough, details which he found in his sources.[312] With strategy we need not concern ourselves, since the subject lies beyond the scope of this essay; but tactics require more consideration on account of the special position assigned to the auxilia in battle formation. From the scanty information given by our authorities it appears that in any regular engagement fought during the first two centuries the legionary infantry were still considered to be the chief arm and employed to deal the decisive blow.[313] They occupied the centre of the line, and the light troops and cavalry—that is to say, the auxilia—were expected to do little more than protect them from a flank attack. This formation was employed at Idistaviso in 16,[314] against Tacfarinas in 18,[315] against Tiridates in 58,[316] against Boudicca in 61,[317] and at the second battle of Bedriacum in 69.[318] It is also prescribed by Arrian in his ‘Order of Battle against the Alani’.[319] The only considerable exception is the battle of Issos in 193, in which the legions on both sides formed the first line and were supported by the archers, who shot over their heads. Dio, however, expressly states that this formation was adopted because these armies were fighting in a narrow space with the sea on one side and mountains on the other, so that there was no need to detach a force to protect their flanks.[320]