The natural base of operations for Kray's army was the Bohemian Mountains and the Enns River, which are about two hundred miles east of the Black Forest. The Austrian lines of communication to this base were over two roads: one by way of Stokach, Memmingen, and Munich; the other along the Danube by way of Mosskirch, Ulm, and Ratisbon. The temporary base of operations for the Austrians in the Black Forest was Ulm. At this place, during the preceding year, the Archduke Charles had constructed an immense intrenched camp.
Knowing that it was necessary to gain a decisive victory over the Austrians in the Black Forest before the Army of Reserve could begin its operations in Italy, the First Consul submitted a plan of campaign which he desired Moreau to carry out. Bonaparte proposed that Moreau should concentrate his forces on the south side of the Rhine between Schaffhausen and Lake Constance, cross the river in force, and attack the flank and rear of the Austrians in the Black Forest. He calculated that, by an attack in this direction, Moreau would be able to defeat Kray, sever his communications, and either capture or destroy his army.
To this plan Moreau objected. It seemed to him a difficult operation. Cautious by nature, he looked upon the proposed manœuvres of Bonaparte as being too bold and hazardous. He argued that his left and centre would have to make long flank marches in order to join his right; and that while the movements were taking place Kray would be given the opportunity of concentrating his forces near Schaffhausen, where he could oppose the passage of the French corps, or crush them in detail as they crossed the river.
The First Consul replied that the Rhine afforded just the kind of protection necessary to screen the French corps during their concentration; and that these manœuvres, if successfully executed, would, in a short time, bring about great results.
But Moreau, who appreciated the difficulties of forcing the passage of a large river in the face of an active enemy, believed that the risk was too great, and therefore refused to attempt the execution of Bonaparte's plan. Instead, he proposed the following plan. His left, under Ste. Suzanne, was to cross the Rhine at Kehl, and his centre, under St. Cyr, at Brisach. Both corps were to push forward, attack Kienmayer, and drive him into the defiles of the Black Forest. Moreau calculated that these attacks would lead Kray to believe that the French forces were massing in front of his right wing, and would cause him to re-enforce Kienmayer. Having driven the Austrians back into the Black Forest, and thus given the impression that the principal attack of the French would be made from the direction of Strasburg, Ste. Suzanne and St. Cyr were to withdraw suddenly. The former was to recross the Rhine at Kehl, ascend the river, cross again to the German side at Brisach, and take the position formerly occupied by St. Cyr; the latter was to make a flank march over the spurs and hills towards Schaffhausen by way of St. Blazien. Meanwhile Moreau with the reserve was to cross the Rhine at Bâle and march towards Schaffhausen, where, upon his arrival, his right, under Lecourbe, was to cross the river and join him. As soon as these movements were completed, Ste. Suzanne was to march towards Lake Constance by way of Friburg, Neustadt, and Loffingen. By this series of complicated manœuvres, Moreau expected to unite the bulk of his forces in the vicinity of Schaffhausen, and to march thence against the flank of Kray in the Black Forest.
Though the First Consul was anxious to have his plan adopted; though he had, in fact, already begun to collect boats in the Rhine preparatory to crossing the river near Schaffhausen, yet Moreau persisted in his own views. Nevertheless, Bonaparte hoped to convince him. With this end in view, he explained the proposed manœuvres and pointed out their advantages to General Dessoles, Moreau's chief of staff. Through this officer, who had an acute intellect and sound judgment, the First Consul hoped to change the views of Moreau himself. Though General Dessoles soon perceived that the plan of Bonaparte was superior to that of Moreau, nevertheless he advised the First Consul to allow Moreau to carry out his own ideas. "Your plan," said he to Bonaparte, "is grander, more decisive, and probably even surer; but it is not adapted to the genius of the man who is to execute it. You have a method of making war which is superior to all others; Moreau has his own,—inferior doubtless to yours, but still an excellent one. Leave him to himself; he will act well, slowly perhaps, but surely; and he will obtain as many results for you as are necessary for the success of your general combinations. If, on the contrary, you impose your ideas on him, you will disconcert him, you will wound his self-love, and obtain nothing from him by seeking to obtain too much." The First Consul appreciated the wisdom of these remarks, coming from such a man, and yielded the point. "You are right," said he to General Dessoles. "Moreau is not capable of grasping and executing the plan that I have conceived. Let him follow his own course; only let him push back Marshal Kray upon Ulm and Ratisbon, and afterwards move his right wing in time upon Switzerland. The plan which he does not understand, and dares not execute, I myself will carry out on another part of the theatre of war. What he dares not attempt on the Rhine, I will accomplish on the Alps."
It being settled that Moreau should proceed against the Austrians in his own way, Bonaparte now wished to come to an understanding with him by which a corps of twenty or twenty-five thousand men should, at the proper time, be detached from the Army of the Rhine, and be sent across Switzerland to unite in Italy with the Army of Reserve. But Moreau did not enter heartily into any of the plans proposed by the First Consul. In fact, both he and Bonaparte seemed to distrust each other. Whether from jealousy, or from honest convictions, Moreau opposed the plans of Bonaparte. Moreover, he had declared that he would not serve under the First Consul, should the latter unite the Army of Reserve with the Army of the Rhine. Naturally this opposition created in the mind of Bonaparte a doubt of Moreau's good faith. He feared that, at the critical moment, the commander of the Army of the Rhine might fail to send a corps into Italy. He was well aware that the commander of an army is always reluctant to weaken his forces after operations have begun; and he knew that circumstances might arise which would seem to justify Moreau in refusing to obey the orders of his superior. He therefore insisted that Moreau should sign a stipulation whereby he promised that, after pushing Kray back from Lake Constance, he would detach Lecourbe with twenty or twenty-five thousand men, and order him into Italy. This agreement was signed at Bâle by Moreau and General Berthier, the latter representing the First Consul.
Nearly a month had passed since the arrangements between Bonaparte and Moreau had been completed. It was now the latter part of April, yet Moreau had made no movement to cross the Rhine and attack Kray. Naturally cautious and slow, he had postponed his advance from day to day, in order, if possible, to supply his army with everything necessary to increase its fighting power. He was short of cavalry and artillery horses, and had little or no camp equipage, and no intrenching tools. But at this time it was impossible for him to obtain everything he needed. Already Bonaparte had sent to the Army of the Rhine all the spare material of war that he could collect in France. Now he was anxious to have Moreau advance. Masséna was hard pressed at Genoa, and Bonaparte desired to march into Italy in order to relieve him. But to cross the Alps and throw himself upon the rear of Melas, while Kray remained undefeated in the Black Forest, was too hazardous an undertaking. Much therefore depended upon the early advance of Moreau. Repeatedly Bonaparte urged him to press forward, and finally sent him a positive order to cross the Rhine and attack Kray.
On the 25th of April Moreau began his movement. Ste. Suzanne crossed the Rhine at Kehl, ascended the Kinzig valley, and pushed Kienmayer's outposts back into the Black Forest. At the same time St. Cyr crossed at Brisach; one division of his corps then advanced towards the Kinzig valley, while the other divisions forced back the Austrians at Friburg, and occupied the entrance to the Höllenthal.
On the next day Kray at Donaueschingen heard of these movements. Having received word that a part of his right wing had been attacked by forty thousand men, he was led to believe that Moreau intended to force his way through the Black Forest by way of the Kinzig valley. He therefore sent seven thousand Austrians from Villingen to re-enforce Kienmayer, and to replace these troops withdrew seven thousand men from his reserve at Stokach. At the same time he ordered his extreme right, under Starray, to move towards the main army into the valley of the Murg.