It was by this manœuvre, also, that, in the campaign of 1809, the Archduke Charles compelled the French to reoccupy the isle of Lobau, after having debouched on the left bank of the Danube. The march of the Archduke Charles was wholly concentric. He menaced Grosaspern with his right, Esling with his centre, and Enzersdorf with his left.
His army, with both wings resting on the Danube, formed a semicircle around Esling. Napoleon immediately attacked and broke the centre of the Austrians; but after having forced their first line, he found himself arrested by the reserves. In the meantime, the bridges upon the Danube had been destroyed, and several of his corps, with their parks of artillery, were still on the right bank. This disappointment, joined to the favorable position of the Austrians, decided Napoleon to re-enter the isle of Lobau, where he had previously constructed a line of field-works, so as to give it all the advantages of a well entrenched camp.
MAXIM XXXVIII.
It is difficult to prevent an enemy, supplied with pontoons, from crossing a river. When the object of an army, which defends the passage, is to cover a siege, the moment the general has ascertained his inability to oppose the passage, he should take measures to arrive before the enemy, at an intermediate position between the river he defends and the place he desires to cover.
NOTE.
Here we may observe, that this intermediate position should be reconnoitred, or rather, well entrenched beforehand; for the enemy will be unable to make an offensive movement against the corps employed in the siege, until he has beaten the army of observation; and the latter, under cover of its camp, may always await a favorable opportunity to attack him in flank or in rear.
Besides, the army which is once entrenched in this manner, has the advantage of being concentrated; while that of the enemy must act in detachments, if he wishes to cover his bridge, and watch the movements of the army of observation, so as to enable him to attack the besieging corps in its lines, without being exposed to an attempt on his rear, or being menaced with the loss of his bridge.