Battle of Quatre Bras in the context of L'Armée du Nord

The Battle of Waterloo was fought on Sunday 18 June 1815, near Waterloo (then in the United Kingdom of the Netherlands, now in Belgium), marking the end of the Napoleonic Wars. The French Imperial Army under the command of Napoleon I was defeated by two armies of the Seventh Coalition. One was a British-led force with units from the United Kingdom, the Netherlands, Hanover, Brunswick, and Nassau, under the command of field marshal Arthur Wellesley, Duke of Wellington. The other comprised three corps of the Prussian army under Field Marshal Blücher. The battle was known contemporaneously as the Battle of Mont Saint-Jean in France (after the hamlet of Mont-Saint-Jean) and La Belle Alliance in Prussia ("the Beautiful Alliance"; after the inn of La Belle Alliance).

Upon Napoleon's return to power in March 1815, the beginning of the Hundred Days, many states that had previously opposed him formed the Seventh Coalition to oppose him again, and hurriedly mobilised their armies. Wellington's and Blücher's armies were cantoned close to the northeastern border of France. Napoleon planned to attack them separately, before they could link up and invade France with other members of the coalition. On 16 June, Napoleon successfully attacked the bulk of the Prussian Army at the Battle of Ligny with his main force, while a small portion of the French Imperial Army contested the Battle of Quatre Bras to prevent the Anglo-allied army from reinforcing the Prussians. The Anglo-allied army held their ground at Quatre Bras but were prevented from reinforcing the Prussians, and on the 17th, the Prussians withdrew from Ligny in good order, while Wellington then withdrew in parallel with the Prussians northward to Waterloo on 17 June. Napoleon sent a third of his forces to pursue the Prussians, which resulted in the separate Battle of Wavre with the Prussian rear-guard on 18–19 June and prevented that French force from participating at Waterloo.

The Brunswick Ducal Field-Corps (German: Herzoglich Braunschweigisches Feldcorps), commonly known as the Black Brunswickers, was a volunteer military unit raised by Frederick William, Duke of Brunswick-Wolfenbüttel during the Napoleonic Wars. The Duke was a strong opponent of Napoleon's occupation of German territory. Formed in 1809 when war broke out between the First French Empire and the Austrian Empire, the unit initially consisted of approximately 2,300 infantrymen and cavalrymen before incorporating a number of artillery troops.

Most members of the unit wore distinctive black uniforms, although some soldiers, such as light infantrymen and uhlans, wore green uniforms. The Black Brunswickers also wore silver totenkopf badges on their hats. Their title originated from the Duke, who claimed Brunswick–Lüneburg, which the French had abolished in order to incorporate its lands into the Kingdom of Westphalia. The unit earned themselves a fearsome reputation over the following decade, taking part in several significant battles, including the Battle of Quatre Bras on 16 June 1815, where the Duke lost his life. However, recruiting, the replacement of casualties, and finance had always been problematic for the unit, which was disbanded in the early 1820s.

An infantry square, also known as a hollow square or square formation, was a musket-era historic close order formation used in combat by infantry units, usually when threatened with cavalry attack. To deploy its weapons effectively, a traditional infantry unit would generally form a line; but the line was vulnerable to more nimble cavalry, which could sweep around the end of the line, or burst through it, and then attack the undefended rear or simply sweep along the line attacking the individual footsoldiers successively. By arranging the unit so that there was no undefended rear or flank, an infantry commander could organise an effective defense against cavalry attack. With both the development of modern repeating firearms and the demise of cavalry in warfare, the square formation is considered obsolete and is effectively never used in modern day warfare.

Infantry tactics are the combination of military concepts and methods used by infantry to achieve tactical objectives during combat. The role of the infantry on the battlefield is, typically, to close with and engage the enemy, and hold territorial objectives; infantry tactics are the means by which this is achieved. Infantry commonly makes up the largest proportion of an army's fighting strength, and consequently often suffers the heaviest casualties. Throughout history, infantrymen have sought to minimise their losses in both attack and defence through effective tactics. (For a wider view of battle and theater tactics see: Military strategy)

Infantry tactics are the oldest method of warfare and span all eras. In different periods, the prevailing technology of the day has had an important impact on infantry tactics. In the opposite direction, tactical methods can encourage the development of particular technologies. Similarly, as weapons and tactics evolve, so do the tactical formations employed, such as the Greek phalanx, the Spanish tercio, the Napoleonic column, or the British 'thin red line'. In different periods the numbers of troops deployed as a single unit can also vary widely, from thousands to a few dozen.