Wellington's defensive position behind a ridge and in buildings was attacked by the French. The very wet ground, a late start to the battle and the dogged resistance of the troops were all in favour of the allies. Napoleon's frontal attacks lacked the necessary coordination of infantry, cavalry and artillery. When Blücher brought his Prussians into action on the French right the French defeat was sealed.
Napoleon's gamble to take the Anglo-Dutch and the Prussian armies separately almost came off. Failure to exploit either of the opening two battles of the campaign at Ligny and Quatre Bras plus Wellington's determined defence at Mont St Jean plus a Prussian Corps at Wavre holding down Grouchy's Corps allowed the Allies, eventually, to act in consort on the battlefield. Napoleon's last throw ended in defeat.