The battle took place near the strategic crossroads of Quatre Bras[a] and was contested between elements of the Duke of Wellington's Anglo-allied army and the left wing of Napoleon Bonaparte's French Armée du Nord under Marshal Michel Ney.

Facing two armies (Wellington's arriving from the west and the Prussians under Field Marshall von Blücher from the east), Napoleon's overall strategy was to defeat each in turn, before these forces could join.

The crossroads at Quatre Bras therefore became a strategic position, since if the French held this interchange, they could prevent Wellington's forces from moving south-eastward along the Nivelles-Namur road towards the Prussians, where Napoleon was planning to engage von Blücher on 16 June at Ligny.

"[3] While Wellington's instructions at the start of the campaign were to defend Brussels from the French, he was not sure what route Napoleon's army would follow; he also received (false) reports of a flanking maneuver through Mons to the southwest.

Within the next few hours, he received additional news that the French had skirmished early that morning with the Prussian First Corps under Lieutenant-General Graf von Zieten at Thuin (near Charleroi).

This nine-hour delay meant it was too late for him to move his army in sufficient strength to provide von Blücher with support on 16 June at the Battle of Ligny.

The headquarters of the Anglo-allied First Corps (Prince of Orange's), however, decided to ignore Wellington's order that it should assemble in and around Nivelles, instead opting to take the initiative and converge on Quatre Bras.

However, once the French were in possession of Sombreffe, Napoleon planned to swing the reserve westwards to join with Marshal Ney, who—it was supposed—would have by that time secured the Quatre Bras crossroad.

[6] Accordingly Marshal Ney, to whom III Cavalry Corps (Kellermann) was now attached, was to mass at Quatre Bras and push an advance guard 10 kilometres (6.2 mi) northward of that place, sending a connecting division at Marbais to link him with Grouchy.

Napoleon now awaited further information from his wing commanders at Charleroi, where he massed the VI Corps (Lobau's), to save it, if possible, from a harassing countermarch, as it appeared likely that it would only be needed for the march to Brussels.

At the headquarters of the I Corps at Genappe (about five kilometres (3 miles) from Quatre Bras), Major-General Jean Victor de Constant Rebecque, chief of staff to the Prince of Orange, realized the danger and ordered Lieutenant-General Hendrik George de Perponcher Sedlnitsky, the commander of the 2nd Dutch Division, to dispatch his 2nd Brigade (Prince Bernhard of Saxe-Weimar-Eisenach) to occupy Quatre Bras.

Prince Bernhard was able to deploy prior to the arrival of the first French scouts, lancers of the Guard Light Cavalry Division (Lefebvre-Desnouettes) who approached Quatre Bras.

[7] By countermanding a direct order from Wellington and using his own initiative, Rebecque was responsible for the Battle being fought at Quatre Bras on the following day, thereby preventing the French from keeping the two coalition armies apart and destroying each of them in detail.

[9] These observations led Wellington to conclude that Napoleon was bringing the main force of his army to bear against the Prussians; he at once proposed to assist von Blücher by first advancing straight upon Frasnes and Gosselies, as soon as he was able to concentrate sufficient force, and then attacking the French from their left and rear, thus providing a powerful diversion to aid the Prussians, since von Blücher's right wing was the weakest and most exposed, and considering Napoleon's movements was the most likely to be attacked.

[b] Siborne, writing from eyewitness accounts, records it thus: "Upon a calculation being made, however, of the time which would elapse ere the Duke would be able to collect the requisite force for undertaking this operation, and of the possibility of Blucher being defeated before it could be carried into effect, it was considered preferable that Wellington should, if practicable, move to the support of the Prussian Right by the Namur road.

The latter having expressed his confident expectation of being enabled to afford the desired support, as also of his succeeding in concentrating, very shortly, a sufficient force to assume the offensive, rode back to Quatre Bras."

Fighting started late in the afternoon on 15 June, when the Elba squadron, a small Polish lancer unit consisting of only 109 men and officers, tried to attack the allied forces from the direction of Frasnes.

The Dutch picket line of the 2nd Division (Sedlnitsky) greeted them with musket volleys, but it was outnumbered and those east of the Brussels highway were at once forced back by the mass of men moved against them.

Regiments of the British 9th Brigade (Pack) – 42nd ("Black Watch", Macara), 44th ("East Essex", Hamerton) and 92nd ("Gordon Highlanders", John Cameron) – held up against the infantry.

At quarter past the hour Ney heard that the French I Corps (d'Erlon), without his direct order or knowledge, had moved eastwards to assist in the battle of Ligny.

Fifteen minutes later at 17:30 he received an unclear order from Napoleon to seize Quatre Bras and then turn eastwards to crush Blücher, who was caught at Ligny.

Due to the arrival of allied reinforcements, Ney realized that he could capture and hold Quatre Bras only with the support of the I Corps and he sent imperative orders to d'Erlon to return at once.

[12] Kellermann's cuirassiers caught the British 5th Brigade (Halkett) – 33rd ("West Riding", Lieutenant-Colonel William George Keith Elphinstone) 69th ("South Lincolnshire", Morice) and the 73rd (Harris) – in line formation.

The 69th were badly mauled, losing their King's colour (the only battalion under Wellington's direct command to do so); the 33rd and the 73rd were saved from a similar fate by running for the safety of Bossu Wood where they rallied quickly.