Wellington and Blucher were well aware that Napoleon would have Brussels firmly in his sights and would be desperate to destroy the Allied armies before they had a chance to gel together. However, they had no idea of the route that Napoleon would take to Brussels since their intelligence was sketchy and they were hardly equipped to cover every combination. Napoleon, on the other hand, was well informed as to the Allied dispositions due to the strong pro-French element in the Netherlands at the time.
He chose to attack the Prussian position first and then swing round to swallow up Wellington
He chose to attack the Prussian position first and then swing round to swallow up Wellington. Napoleon clearly considered the Prussians to be a tougher test for his force, apparently holding Wellington's ability in fairly low regard even though Wellington had defeated every French general he faced in the Peninsular War (1808-1813). The French Army crossed the River Sambre on the 13th June and the advance guard sharply attacked the Prussian I Corps, causing it to fall back from Charleroi. Blucher immediately began concentrating his forces at Sombreffe as planned and sent word to Wellington of the attack. The information Wellington received was vague in terms of the size of the French force and the extent of the attack. He sent out orders for the concentration of his divisions, apparently fearing for his right flank.
On the evening of 15th June, Wellington and his most senior officers attended the Duchess of Richmond's ball. This seems to have been a deliberate attempt on the part of the Duke to maintain an air of normality in the face of the French sympathisers in Brussels and to ensure that his generals were close at hand. During this ball, the news arrived from Blucher that the Prussians had been driven back at Fleurus (close to Sombreffe) and then from the Prince of Orange that French scouts had reached Quatre Bras. This was an important crossroads on the main road between Wellington and Blucher and it immediately became clear that Napoleon intended to split the two armies. Wellington was determined to hold this crossroads and immediately sent troops there. Control of Quatre Bras meant that Wellington could support Blucher without leaving the road to Brussels exposed.