Lower Austria did have a large Romanized population, but it was concentrated along the Danube and in the lower reaches of the fertile Traisen Valley. The only Roman road in Danubian Lower Austria ran through these densely populated riverside areas from Linz to Melk, whence it skirted the steep hills opposite the Wachau and reached the Traisen at Sankt Pölten, then turning once again toward to the Danube at Traismauer, where a cavalry unit was garrisoned in the late imperial period. From this point the road followed the Danube by way of Tulln to Vienna, where it was joined by the routes leading southeast via Baden or Sopron to the important junction at Szombathely. Thus, the connection between Noricum Ripense and Sirmium ran along the Danube to Vienna, then through Pannonia via Szombathely, Lake Balaton, and Pécs. Points along this ancient route to Sirmium appear prominently in the Carolingian sources.
Frankish Bavaria, then, inherited an elaborate system of Roman roads that, following the natural contours of the Alps, led southeast. If Frankish expeditions in the ninth century were directed against a Moravia that lay somewhere near the watergate of the central Danubian basin, Carolingian armies would certainly have utilized these routes and commanders would have taken great pains to establish support facilities along them. Armies are cities in motion, and, as such, they have basic needs that must be satisfied each day. The momentum of an army, like that of a battleship, makes it difficult to change its direction. While armies can maneuver, of course, their freedom of action is always limited by considerations involving the availability of food and fodder; they run great risks when they venture into regions where men and animals cannot be sustained. Evidence of logistical infrastructures along routes leading from Bavaria in the direction of Belgrade provides a powerful argument supporting the southern Moravian hypothesis, for such a military organization would only make sense if Carolingian objectives involved control of the central Danubian watergate.
In a stimulating book on the logistics of the Macedonian army, Donald Engels whose salient findings are also relevant to the age of Charlemagne demonstrated the dimensions of the supply problem that Alexander the Great’s armies faced when invading hostile territory. He concluded that three days was the maximum survival time for an army that had to carry all of its own food, fodder, water, tents, and armor through the most difficult terrain possible. To survive for four days a huge train of pack animals would have been necessary. Even at that, on the fifth day a crisis would occur, for by then men and beasts would have consumed all of the increased burden that the pack animals could have carried, even at half rations. Engels also discovered that the ratio between the army’s consumption rate and its carrying capability remains constant no matter how many personnel or pack animals are used to transport supplies, so a smaller force would find it no easier to carry its own supplies than a larger one. Moreover, it would not help if cavalry mounts packed supplies, for war-horses used in this manner cannot be ready for combat when they arrive at their destination, ”after the fourth day, they would be so much meat on the hoof.” By the fifth day the army would have suffered heavy casualties, leaving it in no condition to conduct a siege or to fight a battle. Thus, Engels limits the effective range of an army operating under such difficult circumstances to three days, or roughly ninety kilometers.
However, this is a worst-case scenario that assumes that there were no provisions along the line of march, that the land was either barren or that it was not harvest time and/or it had been deserted by indigenous peasants who had taken their food supplies with them. This model also posits that there was no potable water along the route. While Alexander the Great, who operated in the arid Near East, dealt with such extremes, Carolingian armies would never have faced similar prospects in the central Danubian basin; thus, they had a greater range. Moreover, it should be obvious that a change in any variable of this model results in a considerable augmentation of the distance that an army could cover before running out of supplies. Engels estimates, for example, that if the army did not have to carry its own water, it could triple its range to nine days or three hundred kilometers. Moreover, large-scale Carolingian military operations against Moravia generally did take place in the summer and were accompanied by the pillaging of territory, so some forage must have been available for troops and animals. The availability of forage would, of course, have considerably increased the distances over which Carolingian armies could strike.
The territory that an army could effectively dominate also varied with the physical features of the landscape. Navigable rivers exponentially increased the range of armies. Greater amounts of food, fodder, and water can be conveyed by boats (which need no nourishment) than by a train of pack or draft animals, each devouring a daily ration of ten pounds of grain and anequal amount of hay, which also had to be transported. 66 Barges moving downstream with the current proceed more quickly than a supply train, and, unlike animals, barges do not tire. The availability of rivercraft would also have facilitated the movement of such heavy equipment as siege engines, tents, arms, armor, picks, shovels, and so on. As we shall see, rivercraft operating on the Danube, the Drava, and the Sava considerably eased the problem of supply in the central Danubian basin. Terrain also determined the rate of march, a significant consideration for an army carrying all or even a portion of its supplies. Because daily rates of consumption are invariable, it is important for an army on campaign to reach its destination or some point of resupply as quickly as possible.
Assuming the validity of a southern Moravian thesis, there were four routes from Bavaria to the Danubian watergate. One, of course, was the Danube itself. This route would have been the best assuming that Rastislav’s realm was centered on the Alföld. There were, however, some strategic problems with it, involving suitable localities for troops to disembark between Budapest and Belgrade. Also, river convoys could have been easily ambushed unless they were supported by terrestrially based forces moving along both banks of the river. Such forces would have had to endure a long and difficult march. And, once troops and supplies had landed, they still faced a march of several days over difficult terrain that would have favored the indigenous defenders before reaching the centers of a Moravian realm east of the Tisza. Assuming that Frankish forces landed near either Osijek or Novi Sad, they would have had two hundred kilometers in front of them (seven days) to reach Cenad, if that is where Rastislav’s capital was located on the Alföld. If they did not have to carry water, such an objective could be achieved. Nonetheless, the army would have been exhausted and isolated from its sources of support before arriving at its destination. A final difficulty involved the necessity of leaving some forces behind to guard the rivercraft. As we shall see, such forces and their boats were extremely vulnerable to surprise attacks. On strictly theoretical grounds, then, Eggers’s conclusion that it would have been difficult for Carolingian forces to have exercised a great deal of control over a Moravian principality east of the Tisza is plausible.
A second route (a land one) was the most direct. It went diagonally through modern Hungary from Sopron to Pécs. This route would have been suitable for campaigns against a realm located either on the Alföld or near Sirmium. From Vienna this route followed the Roman road to Szombathely, then skirted the swamps west of Lake Balaton and veered south-eastward in the direction of Pécs. An army using this route would have had to transport its supplies or otherwise acquire them along the way. Since this route through Hungary was used by many crusading armies in a later and better documented era, we have some knowledge of the rates of march of those forces. During the Third Crusade, for example, Frederick Barbarossa’s army arrived in Vienna on May 26 or 27, but only reached Belgrade on June 29. This army, however, encountered frequent delays, so we need not assume that Carolingian units would have required an entire month to traverse four hundred kilometers from Vienna to Belgrade. The crusaders encountered different circumstances in the more developed twelfth century than those which Carolingian armies had to face in the ninth. For example, crusaders purchased their comestible supplies along the way. This fact did not, however, increase their rate of march; on the contrary, many of the delays experienced by these forces resulted from problems and expenses involved in the procurement of necessities. Yet Barbarossa’s army sometimes advanced at a rate of 18.8 miles per day, which roughly corresponds to Engels’s estimated daily average of thirty kilometers. On the other hand, it is probable that Carolingian armies on a prolonged march through Pannonia could have averaged no more than twenty-five kilometers.
If we assume that Carolingian forces were able to maintain a pace of twenty-five kilometers each day on the march through Pannonia, it still would have taken them fifteen days to reach Sirmium from Vienna (between 350 and 380 kilometers). Thus, Sirmium was well beyond the range of a self-sufficient army that had to carry its supplies from bases near Vienna. Frankish forces using this route could, then, only have invaded the region bounded by the lower reaches of the Drava and the Sava if there were adequate support facilities along the way. A Carolingian army operating from Szombathely, on the other hand, would have been 85 kilometers closer (approximately eleven days from Sirmium); a base near the western end of Lake Balaton would have been 65 kilometers closer still (within the nine days); and a place of resupply at Pécs would have been only 120 kilometers (five days) from the ancient capital on the lower Sava. As we have seen, a force that did not have to carry its own water could sustain itself for nine days. Thus, an army moving through Pannonia and supported by existing facilities as far as Pécs (or even Zalavár) would have been within striking distance of Sirmium, even if it had to carry its own food and fodder from that point on. This task, however, would not have been necessary, for, southeast of Pécs, forces could have been refurbished by boats operating on the Drava and Sava rivers.
Since an army marching overland from Sopron to Sirmium via Pécs could be resupplied by rivercraft on the Drava at Osijek and provisions and siege engines could be transported on the Sava to the very walls of Sirmium itself, we must conclude that the ancient capital was within reach of an army that began its march to the southeast from Vienna, provided that the key points of supply in Pannonia (Sopron, Szombathely, Zalavár, and Pécs) were in the hands of Frankish margraves or indigenous Slavic leaders who supported them and that the upper reaches of the Drava and Sava in Carantania were in friendly hands. From localities in Transdanubia the troops could be refurbished with food, fodder, and potable water. Siege and noncomestible equipment could be ferried down the Drava and Sava in the company of troops entering Pannonia from bases in Carantania. If Carolingian armies were indeed able to conduct effective military operations against a Moravian principality situated between the lower Drava and Sava, or one east of the Danube and the Tisza, there must have existed a logistical infrastructure extending along the overland route from Vienna to Szombathely and extending via Zalavár as far southeast as Pécs. A point of supply near the west end of Lake Balaton (Zalavár), two and a half days from Szombathely, would have eased considerably the movement of an army in the direction of Pécs, approximately one hundred kilometers farther southeast.
The pivotal strongholds in such a system of logistics would have been Sopron and Szombathely, where the hills of Burgenland give way to the rolling country of western Hungary. Szombathely would have been especially important. In addition to controlling the road leading southeast to Balaton and Pécs, it also straddled the ancient via militaris leading southwest to the Drava crossing at Ptuj, a march of four days (slightly more than one hundred kilometers). Ptuj, the locality that would have ensured communications between forces moving southeast from Carantania with those operating in Pannonia, was also a point from which the Pannonian army could have been resupplied by rivercraft on the Drava. A communication and supply system linking forces advancing through Pannonia with those moving along the Drava and Sava from Carantania would have been indispensable to support armies invading the region near Sirmium.
This discussion leads us to a consideration of the two remaining routes. These were the ones leading from Bavaria through the Alps to the region known in ninth-century sources as Carantania (the modern Austrian provinces of Carinthia and Styria and the South Slavic Republic of Slovenia), whence one route followed the Drava and the other the Sava south-east. Although these rivers could only be reached after grueling marches over passes, once their headwaters had been attained, the routes along them had an advantage insofar as fluvial navigation could then be utilized to supply armies proceeding southeast. Utilizing the Drava and the Sava, armies could advance rapidly toward the central Danubian watergate. Control over these two routes was, then, absolutely essential for Frankish military operations against a Moravian principality located in southeastern Pannonia. As we shall see, Carolingian expeditions against the realms of Rastislav and Zwentibald had little chance of success when East Frankish kings lost control of Carantania.