On 13 March 1815, six days before Napoleon reached Paris, the powers at the Congress of Vienna declared him an outlaw; four days later, the United Kingdom, Russia, Austria, and Prussia bound themselves to put 150,000 men each into the field to end his rule.
Napoleon knew that once his attempts at dissuading one or more of the Seventh Coalition Allies from invading France had failed, his only chance of remaining in power was to attack before the Coalition could put together an overwhelming force.
If he could destroy the existing Coalition forces south of Brussels before they were reinforced, he might be able to drive the British back to the sea and knock the Prussians out of the war.

The Duke of Wellington expected Napoleon to try to envelop the Coalition armies, a maneuver that he had successfully used many times before, by moving through Mons to the south-west of Brussels.
The roads to Mons were paved, which would have enabled a rapid flank march.
This would have cut Wellington's communications with his base at Ostend, but would also have pushed his army closer to Blücher's.
In fact, Napoleon planned instead to divide the two Coalition armies and defeat them separately, and he encouraged Wellington's misapprehension with false intelligence.
Moving up to the frontier without alerting the Coalition, Napoleon divided his army into a left-wing, commanded by Marshal Ney, a right-wing commanded by Marshal Grouchy, and a reserve, which he commanded personally (although all three elements remained close enough to support one another).
Crossing the frontier at Thuin near Charleroi before dawn on 15 June, the French rapidly overran Coalition outposts and secured Napoleon's favored "central position" – at the junction between the area where Wellington's allied army was dispersed to his north-west, and Blücher's Prussian army to the north-east.

Only very late on the night of 15 June was Wellington certain that the Charleroi attack was the main French thrust, and he duly ordered his army to deploy near Nivelles and Quatre Bras.
Early on the morning of 16 June, at the Duchess of Richmond's ball, on receiving a dispatch from the Prince of Orange, he was shocked by the speed of Napoleon's advance, and hastily sent his army in the direction of Quatre Bras, where the Prince of Orange, with the brigade of Prince Bernhard of Saxe-Weimar, was holding a tenuous position against the French left, commanded by Marshal Ney.}
Ney's orders were to secure the crossroads of Quatre Bras so that if necessary, he could later swing east and reinforce Napoleon.

As Napoleon considered the concentrated Prussian army the greater threat, he moved against them first.
Lieutenant-General Zieten's I Corps rearguard action on 15 June held up the French advance, allowing Blücher to concentrate his forces in the Sombreffe position, which had been selected earlier for its good defensive attributes.

Napoleon's original plan for 16 June was based on the assumption that the Coalition forces, who had been caught napping, would not attempt a risky forward concentration; and he intended, therefore, to push an advance guard as far as Gembloux, to feel for and ward off Blücher.
To assist this operation the reserve would move at first to Fleurus to reinforce Grouchy, should he need assistance in driving back Blücher's troops; but, once in possession of Sombreffe, Napoleon would swing the reserve westwards and join Ney, who, it was supposed, would have in the meantime mastered Quatre Bras.

In pursuance of this object Ney, to whom III Cavalry Corps (Kellermann) was now attached, was to mass at Quatre Bras and push an advanced guard 10 kilometres (6.2 mi)  northward of that place, with a connecting division at Marbais to link him with Grouchy.
The centre and left-wing together would then make a night-march to Brussels.
The Coalition forces would thus be irremediably sundered, and all that remained would be to destroy them in detail.
Napoleon now awaited further information from his wing commanders at Charleroi, where he massed the VI Corps (Lobau), to save it, if possible, from
On 13 March 1815, six days before Napoleon reached Paris, the powers at the Congress of Vienna declared him an outlaw; four days later, the United Kingdom, Russia, Austria, and Prussia bound themselves to put 150,000 men each into the field to end his rule. Napoleon knew that once his attempts at dissuading one or more of the Seventh Coalition Allies from invading France had failed, his only chance of remaining in power was to attack before the Coalition could put together an overwhelming force. If he could destroy the existing Coalition forces south of Brussels before they were reinforced, he might be able to drive the British back to the sea and knock the Prussians out of the war.
The Duke of Wellington expected Napoleon to try to envelop the Coalition armies, a maneuver that he had successfully used many times before, by moving through Mons to the south-west of Brussels. The roads to Mons were paved, which would have enabled a rapid flank march. This would have cut Wellington's communications with his base at Ostend, but would also have pushed his army closer to Blücher's. In fact, Napoleon planned instead to divide the two Coalition armies and defeat them separately, and he encouraged Wellington's misapprehension with false intelligence. Moving up to the frontier without alerting the Coalition, Napoleon divided his army into a left-wing, commanded by Marshal Ney, a right-wing commanded by Marshal Grouchy, and a reserve, which he commanded personally (although all three elements remained close enough to support one another). Crossing the frontier at Thuin near Charleroi before dawn on 15 June, the French rapidly overran Coalition outposts and secured Napoleon's favored "central position" – at the junction between the area where Wellington's allied army was dispersed to his north-west, and Blücher's Prussian army to the north-east.
Only very late on the night of 15 June was Wellington certain that the Charleroi attack was the main French thrust, and he duly ordered his army to deploy near Nivelles and Quatre Bras. Early on the morning of 16 June, at the Duchess of Richmond's ball, on receiving a dispatch from the Prince of Orange, he was shocked by the speed of Napoleon's advance, and hastily sent his army in the direction of Quatre Bras, where the Prince of Orange, with the brigade of Prince Bernhard of Saxe-Weimar, was holding a tenuous position against the French left, commanded by Marshal Ney.} Ney's orders were to secure the crossroads of Quatre Bras so that if necessary, he could later swing east and reinforce Napoleon.
As Napoleon considered the concentrated Prussian army the greater threat, he moved against them first. Lieutenant-General Zieten's I Corps rearguard action on 15 June held up the French advance, allowing Blücher to concentrate his forces in the Sombreffe position, which had been selected earlier for its good defensive attributes.
Napoleon's original plan for 16 June was based on the assumption that the Coalition forces, who had been caught napping, would not attempt a risky forward concentration; and he intended, therefore, to push an advance guard as far as Gembloux, to feel for and ward off Blücher. To assist this operation the reserve would move at first to Fleurus to reinforce Grouchy, should he need assistance in driving back Blücher's troops; but, once in possession of Sombreffe, Napoleon would swing the reserve westwards and join Ney, who, it was supposed, would have in the meantime mastered Quatre Bras.
In pursuance of this object Ney, to whom III Cavalry Corps (Kellermann) was now attached, was to mass at Quatre Bras and push an advanced guard 10 kilometres (6.2 mi)  northward of that place, with a connecting division at Marbais to link him with Grouchy. The centre and left-wing together would then make a night-march to Brussels. The Coalition forces would thus be irremediably sundered, and all that remained would be to destroy them in detail. Napoleon now awaited further information from his wing commanders at Charleroi, where he massed the VI Corps (Lobau), to save it, if possible, from
Battle of Ligny