Napoleon, therefore, attacking Charleroi suddenly, imagined that he would have to deal only with the First Corps at Charleroi and its neighbourhood. He did not think that the other three corps had information in time to enable them to come up westward towards the end of the line and meet him. The outposts of the First Corps had, of course, fallen back before the advance of the Emperor's great army; the mass of that First Corps was, he knew, upon this morning of the 16th, some mile or two north and east of Fleurus, astraddle of the great road which leads from Charleroi to Gembloux. At the very most, and supposing this First Corps (which was of 33,000 men, under Ziethen) had received reinforcements from the nearest posts of the Second and the Third Corps, Napoleon did not think that he could have in front of him more than some 40,000 men at the most. He was in error. It had been arranged among the Prussian leaders that resistance to Napoleon, when occasion might come for it, should be offered in the neighbourhood of the cross-roads where the route from Charleroi to Gembloux crosses that from Nivelles to Namur. In other words, they were prepared to stand and fight between Sombreffe and the village of Ligny. The plan had been prepared long beforehand. The whole of the First Corps was in position with the morning, awaiting the Emperor's attack. The Second Corps had been in motion for hours, and was marching up during all that morning. So was the Third Corps behind it. Blucher himself had arrived upon the field of battle the day before (the 15th), and had written thence to his sovereign to say that he was fully prepared for action the next day. Indeed, Blucher on the 15th confidently expected victory, and the end of the campaign then and there. He had a right to do so, for Napoleon's advance had been met by so rapid a concentration that, a little after noon on that Friday the 16th, and before the first shots were fired, well over 80,000 men were drawn up to receive the shock of Napoleon's right wing. But that right wing all told, even when the belated French troops beyond the Sambre had finally crossed that river, and even when the Emperor had brought up the Guard and the reserve, numbered but 63,000. Supposing the French had been able to use every man, which they were not, they counted but seven to nine of their opponents. And the nine were upon the defensive; the seven had to undertake the task of an assault.