In case the archduke opposed the crossing, it was vital for the French to establish bridgeheads in the two villages on the farther bank. Both had good defensive features, being encircled by earth embankments to keep out floods, and they were connected to each other by a trench. Most of their houses were built of stone. The one, Aspern, had several streets and a cemetery surrounded by a stout wall. The other, Essling, had only one street, but its granary was a three-story structure of brick, 36 meters by 10, proof against cannon shots up to the first story and big enough to house 400 men.

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On the evening of May 13, Napoleon told Massena to organize the Ebersdorf bridging operation in liaison with his corps artillery commander, General Pernetti, and the army’s chief engineer, General Henri-Gatien Bertrand. Massena was an old hand at crossing rivers–10 years earlier, in a blizzard, he had crossed the Upper Rhine when it was in flood by building a bridge of local timber, personally supervising his sappers as they worked in ice-cold water up to their necks.

The first stage of the operation would be to lay a bridge of boats over the first arm of the Danube to Lobau. As soon as this was done, the advance guard and Lasalle’s light cavalry would pass over into Lobau, together with the material needed to bridge the Stadlau arm to the left bank. The bridging system the French had chosen entailed anchoring a line of flat-bottomed, sheer-sided boats at well-defined intervals and covering them with wooden planks. If the anchoring and spacing were properly done, such a bridge would support the weight of mounted regiments, artillery field pieces and closedup infantry columns marching in fours, at an average rate of passage of 6,000 or 7,000 men per hour.

To throw such a bridge across the Danube at Vienna called for many hours of backbreaking work, but the French pontonniers were used to that; in Napoleon’s army the basic bridging unit, the bateau gribeauval, was more than 36 feet long by more than 4 feet high and weighed more than 4,000 pounds.

As the length of bridge covered by each boat was 32 feet, 80 boats would be needed for the section between the Vienna bank and Lobau. Bertrand already had 48 boats in good repair, and another 32 which he thought could be made ready by the following night; the work would require a great deal of material, including 3,000 beams, 400 joists and 5,000 to 6,000 fathoms of rope. The second arm of the river, the Stadlau Branch, would be bridged by three trestles and by 15 pontoons captured from the Austrians at Landshut.

By the 17th, 91 boats had been assembled, 70 of which had rigging, oars and accessories. Twelve proved to be too heavy; 38 were suitable for floating supports and 20 more could be made so while the bridging was still in progress.

Since he was committing his army to the passage of a great river on a line of hastily assembled boats, rafts, trestles and pontoons, Napoleon was taking a tremendous risk by providing neither cruising vessels nor a boom to protect against enemy fireships. But there was an even greater danger, one which Napoleon may have failed to understand at all.

When the French army had crossed the Danube in 1805, it had been late autumn. The bridges at Vienna were intact. There had been no need to take account of the effect that melting snows might have on the river. In 1809, according to the artillery general, Baston, Comte de Lariboisiere, there was even less cause for concern since the weather was good and there-was no sign of a storm.

But it was precisely the fair weather that made Napoleon’s plan so hazardous. It was no use basing plans or theories on the behavior of the Rhine, which melting snows raised no more than a foot or so. The Danube was very different. Of its 400 tributaries, many came from the Swiss or Tyrolese uplands and the Bavarian Alps. In May and June, the melting snows from these regions could raise the Danube at Vienna by as much as 15 feet- already that spring of 1809, the level had varied from 4 feet above an extreme low-water mark to 13 feet below flood level. When the river reached its maximum height, each of its arms became a miniature sea in which islets and sandbanks disappeared and trees torn from the river banks would sweep downstream on the torrent.

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