Taking advantage of the Prussians' retreat, Napoleon decided it was time to launch a decisive counterstrike.
He could at least beat Blücher and render the Prussians unfit for any serious operation except retreat on 17 June, although he could no longer expect to destroy the Prussian army.
Lobau's VI Corps, too, was now arriving and forming up on the heights east of Fleurus.
The artillery of the Guard, therefore, came into action above Ligny to prepare Blücher's centre for assault.
Some delay was occasioned by a thunderstorm; but, as this passed over, the guns opened and the Old Guard, supported by the reserve cavalry – the Grenadiers à Cheval de la Garde Impériale – led by Guyot, as well as Milhaud's IV Cavalry Corps proceeded to form up opposite Ligny.
At about 19:45 a crashing salvo of 60 guns gave the signal for a combined assault to be delivered by Gerard and the Guard, with Milhaud's cavalry moving on their right flank.
Initially, the French Guard encountered heavy resistance, and was forced back momentarily by the Prussian reserves.
 However Blücher's worn-out soldiers at that section of the line could not withstand the concentrated impact of Napoleon's choicest troops, combined with a flanking movement by a division of French infantry under cover of the darkness, and at around 20:30 the Prussian centre at Ligny was overwhelmed.

The Prussians formed a new defensive line between Brye and Sombreffe, on the heights about 1.6 kilometres (1 mi) behind Ligny.
Units of the I Corps and II Corps retreated to this new position and rallied, fending off French attacks as they did so.

In the words of William Siborne:
The Prussian infantry compelled to evacuate Ligny, effected its retreat in squares, in perfect order, though surrounded by the enemy, bravely repelling all further attacks, made in the repeated but vain attempts to scatter it in confusion.
Taking advantage of the Prussians' retreat, Napoleon decided it was time to launch a decisive counterstrike. He could at least beat Blücher and render the Prussians unfit for any serious operation except retreat on 17 June, although he could no longer expect to destroy the Prussian army. Lobau's VI Corps, too, was now arriving and forming up on the heights east of Fleurus. The artillery of the Guard, therefore, came into action above Ligny to prepare Blücher's centre for assault. Some delay was occasioned by a thunderstorm; but, as this passed over, the guns opened and the Old Guard, supported by the reserve cavalry – the Grenadiers à Cheval de la Garde Impériale – led by Guyot, as well as Milhaud's IV Cavalry Corps proceeded to form up opposite Ligny. At about 19:45 a crashing salvo of 60 guns gave the signal for a combined assault to be delivered by Gerard and the Guard, with Milhaud's cavalry moving on their right flank. Initially, the French Guard encountered heavy resistance, and was forced back momentarily by the Prussian reserves.  However Blücher's worn-out soldiers at that section of the line could not withstand the concentrated impact of Napoleon's choicest troops, combined with a flanking movement by a division of French infantry under cover of the darkness, and at around 20:30 the Prussian centre at Ligny was overwhelmed.
The Prussians formed a new defensive line between Brye and Sombreffe, on the heights about 1.6 kilometres (1 mi) behind Ligny. Units of the I Corps and II Corps retreated to this new position and rallied, fending off French attacks as they did so.
In the words of William Siborne:
The Prussian infantry compelled to evacuate Ligny, effected its retreat in squares, in perfect order, though surrounded by the enemy, bravely repelling all further attacks, made in the repeated but vain attempts to scatter it in confusion.
Battle of Ligny