Julius Caesar: Building Bridges

Most people know that Julius Caesar bridged the Rhine in 55 and 53 BC, and he goes into immense detail on just exactly how he did it (Gallic War 4.17-19 and 6.6, 9, 29, 35). His example was then followed at various points during the next century, especially by his lieutenants. What is remarkable, however, is that most often when the bridging of a river was necessary prior to and after Caesar, it was done, instead, using other methods or via a bridge of boats. This makes what Caesar did, how he did it, and the meticulous detail in which he recorded what he did, all the more noteworthy.   

Famously, Herodotus describes Xerxes’ crossing the Hellespont in 480 BC, building a bridge of boats between Abydos and Sestos (Herodotus 7.21.2, 7.25.1, and then the lengthy account of the crossing itself (complete with digressions) 7.34-57). We are told that Xerxes’ army took ‘seven days and seven nights’ to cross “with no pause” (7.56.1). We can contrast this with the rapidity of Caesar’s army’s march in his description (see below).

Alexander’s crossing of the Hellespont in 334 BC (Arrian Anabasis 1.11.6) was managed, not by a bridge at all, but by Parmenion ferrying the troops across in 160 triremes and “a good number of cargo boats.” Thereafter, several of Alexander’s battles involved the contested crossing of a river (the Granicus, Issus, Jaxartes, Hydaspes) and in each case, Alexander seems to have used a ford and fought his way across; no combat bridge building for him. Of course, it is a slightly different thing to build a bridge in potentially hostile territory than it is to attempt a river crossing in the heat of battle and Alexander did, indeed, make use of a bridge of boats on occasion. When Darius fled towards Thapsacus after the battle of Gaugamela in 331 BC, pursued by Alexander, Alexander had two bridges of boats built over the Euphrates (Arrian Anabasis 3.7). When, in 329 BC he marched to the Oxus, Alexander used tent covers filled with chaff stitched together (Anabasis 3.29.3-4). This was also the method used to cross the Jaxartes later in 329 BC (Anabasis 4.4.2-3) and used in combination with boats when crossing the Acesines (the modern Chenab) in 326 (Anabasis 5.20.8-9). Alexander bridged the River Indus using a bridge of boats (Diodorus 17.86.3) and Arrian does include a passage on making a bridge of boats although it is clear Arrian has been influenced by subsequent methods he himself used in the AD 130s (Anabasis 5.7.1-5).

When Hannibal was marching towards Italy in 218 BC, and he reached the Rhône River in southern France, he found he needed to construct a means by which his elephants could cross. He did this by constructing rafts (Livy 21.28.4-12, Polybius 3.45.6-46.12). In both sources we are not told how the men crossed; the method described was only for the benefit of the elephants, possibly on the erroneous understanding that they could not swim. Both Polybius and Livy explain that elephants cannot swim while, in fact, they can, and do swim well (and, presumably, the mahouts would have known the capabilities of their beasts).

In 55 BC, however, Caesar determined that he needed to make a show of strength across the Rhine. Crossing by ships “he neither deemed to be sufficiently safe, nor considered consistent with his own dignity or that of the Roman people” (4.17.1). So, too, was a bridge of boats not contemplated. Despite the inherent difficulty of building a bridge across the Rhine due to a number of factors (breadth, current, depth), Caesar decided to build a bridge according to the following plan (4.17.3-10): “He joined together at the distance of two feet, two piles, each a foot and a half thick, sharpened a little at the lower end, and proportioned in length, to the depth of the river. After he had, by means of engines, sunk these into the river, and fixed them at the bottom, and then driven them in with rammers, not quite perpendicularly, like a stake, but bending forward and sloping, so as to incline in the direction of the current of the river; he also placed two [other piles] opposite to these, at the distance of forty feet lower down, fastened together in the same manner, but directed against the force and current of the river. Both these, moreover, were kept firmly apart by beams two feet thick (the space which the binding of the piles occupied), laid in at their extremities between two braces on each side, and in consequence of these being in different directions and fastened on sides the one opposite to the other, so great was the strength of the work, and such the arrangement of the materials, that in proportion as the greater body of water dashed against the bridge, so much the closer were its parts held fastened together. These beams were bound together by timber laid over them, in the direction of the length of the bridge, and were [then] covered over with laths and hurdles; and in addition to this, piles were driven into the water obliquely, at the lower side of the bridge, and these, serving as buttresses, and being connected with every portion of the work, sustained the force of the stream: and there were others also above the bridge, at a moderate distance; that if trunks of trees or vessels were floated down the river by the barbarians for the purpose of destroying the work, the violence of such things might be diminished by these defenses, and might not injure the bridge.”

Caesar adds (4.18.1) that “within ten days after the timber began to be collected, the whole work was completed, and the whole army led over.” He marched his army into the territory of the Sigambri but they withdrew, and Caesar burned their villages and cut down their crops before proceeding to the land of the Ubii who became allies (4.19.1-3). Learning that the Suevi were assembling an army against him, rather than stay and fight them, Caesar withdrew “having already accomplished all these things on account of which he had resolved to lead his army over, namely, to strike fear into the Germans, take vengeance on the Sigambri, and free the Ubii from the invasion of the Suevi, having spent altogether eighteen days beyond the Rhine , and thinking he had advanced far enough to serve both honour and interest, he returned into Gaul, and cut down the bridge” (4.19.4).

This passage has been much discussed, dissected, and analysed. Here, however, it is worth noting that this bridging method seems to have been new and innovative. It is certainly described in such detail and presented so meticulously to suggest it was an innovation and a novelty. In earlier campaigns, we find the pontoon and raft building the go-to method of making a bridge in war-time, especially in hostile territory. Fording or swimming rivers (as we shall see) remained the most common method of crossing a river. We find in book 6 of Caesar, however, that his (new) method of bridge building had apparently become more common.

After Caesar, we find others following his example - his legate, Caius Fabius, and quaestor, Marcus Crassus, “hastily constructed some bridges” (6.6.1) and Caesar built another bridge into the territory of the Treverii (6.9.3-4). At 6.35.6 we are told that the Sigambri crossed the Rhine “in ships and barks” (navibus ratibusque) nearly fifty kilometres downstream of where Caesar had built his (second) bridge. Clearly, using boats to cross the Rhine was still an option and would remain so – for many of the subsequent invasions across the Rhine and Danube (down even to the sixth century) we are told the tribes crossed but usually without specific details on how – that they crossed in boats of various kinds is the most likely. On occasion we are told that an invading force crossed a frozen river in the late winter/early spring. What is more, in the period after Caesar we seem to find a return to the use of boats and swimming as a means of crossing rivers.

When Aulus Plautius crossed the Medway in Kent in AD 43, during the invasion of Britain, he did not even attempt to build a bridge; he sent his Batavians to swim across and the rest of his troops forded the river (Dio 60.20.2-4). The Batavian auxiliary cohorts can first be found swimming rivers in Germanicus’ campaigns against the Cherusci in AD 16. Germanicus built bridges too, however, and probably on the model of Caesar (Tacitus Annals 2.8). When Suetonius Paulinus was campaigning in northern Wales in AD 61, he sent his troops across the straits in flat-bottomed barges although the Batavian cavalry swam with their horses (Tacitus Annals 14.29). Paulinus was recalled from the conquest of the island of Mona by the revolt of Boudicca, but Gnaeus Julius Agricola returned to the task in AD 78, he again took advantage of swimming auxiliaries (Tacitus Agricola 18).

Only a century after Caesar, however, we find recourse to earlier methods. During Civilis’ revolt in AD 70 we are told Cerialis did not have the boats to build a bridge (another kind of bridge was not contemplated - “the Roman army could not be got across the river in any other way” (Tacitus Histories 5.19). Jordanes records (Getica 77-78) that Cornelius Fuscus crossed the Danube in AD 86 via a bridge of boats and, just as is recorded on Trajan’s Column (in 101 and 104) and the Column of Marcus Aurelius thereafter (in AD 172 and 175), bridges of boats became the most common method of crossing rivers. It was an iconography Marcus repeated on his coinage (BMC RE 4.1427). A preserved fragment of Cassius Dio describes the technique the Romans used in great detail (71.3). Such methods continue into the third and fourth centuries and beyond. Julius Caesar’s innovative combat bridge building was there to be admired as literature but no longer emulated in the field. Caesar’s bridge building was, it seems, more remarkable than even admirers may have thought.   

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