Exhausted by protracted war, France could not match the manpower arrayed against her. Napoleon realised that his best chance lay in striking decisively before weight of numbers could be brought to bear, and resolved to engage enemy forces garrisoned in the Netherlands. 128,000 strong, the French Armée du Nord crossed the frontier on 15 June. Moving swiftly, Napoleon achieved his first strategic goal the following day when French forces engaged both the British and their allies at Quatre Bras and the Prussian army at Ligny, before his opponents could come together. The two battlefields lie seven miles apart.
Crucially, however, Napoleon failed in his second objective, to concentrate his forces against the Prussians and destroy them, leaving the way clear to deal subsequently with Wellington’s army. Marshal Ney, facing the British and their allies on the French left at Quatre Bras, was unable to support Napoleon at Ligny. After bloody fighting, the Allies and the Prussians fell back towards Brussels on 17 June, following parallel lines of retreat. The two Coalition armies maintained contact, while Napoleon failed to capitalise on his victory at Ligny.