Napoleon had practically a choice between two plans. He might invade Belgium on the west, opposite the right of Wellington's widely divided army, and by advancing northwards cut Wellington off from his communication with the sea. This was what Wellington expected; he was very anxious about his supplies, being probably more than doubtful whether his army would find subsistence if compelled to depend on the Prussians, who had quite enough to do in supplying their own army. A corresponding movement on the left of the Prussians was obviously possible, but for many reasons not worth Napoleon's while. The alternative plan, which he adopted, and which all critics consider to have been the best open to him, was to concentrate his army due south of Brussels, at the nearest point to that capital, and cross the Sambre at Charleroi, which would bring him on the point where the English and Prussian armies met. Napoleon knew that the Prussian forces were less dispersed, and generally nearer the frontier than those of Wellington. He therefore calculated that if, as was probable, Blucher concentrated his army for battle, it would be at a point comparatively near to Charleroi, and that Wellington could not be in time to give him serious assistance. He further calculated that Blucher, if defeated, would retreat eastwards in the direction of his proper line of communication, and that then Wellington ought to be an easy prey to the French army, superior in both numbers and quality. Accordingly Napoleon issued orders that his whole army should move at dawn on June 15, and cross the Sambre at or near Charleroi. At the same time he ordered slight demonstrations to be made much further west, in the neighbourhood of Mons, in order that Wellington might be kept as long as possible in doubt as to what Napoleon's real purpose was. Ziethen, who commanded the Prussian corps nearest to Charleroi, had unaccountably taken no steps to destroy the bridges over the Sambre, which would have delayed the French greatly: but he disputed their advance with much skill and pertinacity, and slowly retired north-eastwards to Fleurus. Blucher had on the night of June 14, on receiving certain tidings that the French were in great force beyond the Sambre, ordered his whole army to concentrate at Sombref, four or five miles behind Fleurus, at the point where the road from Charleroi crosses the great high-road that runs a little north of west from Namur to Nivelles, and thence towards the coast. It was at this point that it had been agreed between the allies, some weeks before, that the Prussians should concentrate in case of an advance by Napoleon, which they then thought improbable. One of the four Prussian corps, that of Bulow, was at a great distance, and failed to arrive in time; but those of Pirch and Thielemann duly joined Ziethen. On the 16th Blucher, with nearly 90,000 men, took up a position at Ligny, a mile or so south of the great Namur road, and awaited attack.