Karl Wilhelm Ferdinand, duke of Brunswick (1735-1806), was an experienced military leader who proved to be unable to cope with the armies of both Revolutionary and Napoleonic France, suffering key defeats at Valmy in 1792 and Auerstädt in 1806.
Karl Wilhelm Ferdinand was the son of Karl, duke of Brunswick. His military career began in 1757, when he served under the Duke of Cumberland. He then transferred to the service of his uncle, Ferdinand of Brunswick, one of Frederick the Great's most able generals. He then went on to serve as a Field Marshal in the Prussian army of Frederick the Great, another of his uncles. He fought during the Seven Years War, and was promoted to general in 1773 and to Field Marshal in 1787. In the same year he led a Prussian army that invaded the Netherlands to restore the House of Orange.
He married the daughter of Frederick, Prince of Wales, and their son Freidrich Wilhelm would gain an impressive reputation as a dedicated opponent of France. Karl Wilhelm inherited the duchy of Brunswick after his father's death in 1780, and gained a reputation as an 'enlightened sovereign'.
As war with France became increasingly likely after the French Revolution the anti-French coalition formed a large Prussian, Austrian and Allied army, with 40,000 Prussians and 30,000 Austrians. The Duke of Brunswick was appointed as commander of this army, although his position was made more complex by the presence of King Frederick William II of Prussia with the army (the king's son and heir, the future Frederick William III, was also present with the army, commanding a brigade). The two men had different plans for 1792. Brunswick wanted to use the year to establish a firm base on the French frontier, and then penetrate into France in 1793. The King wanted to invade immediately, and unsurprisingly got his way.
The Allied army crossed the French border on 19 August, but it advanced slowly. Longwy fell to the invaders on 23 August, followed by Verdun on 2 September, but the French were quick to react. One army led by General Kellermann and part of another under General Dumouriez moved to oppose the invasion, and managed to get between the Allied army and the French border. If Brunswick had made a dash for Paris then the city may well have fallen, but he was unwilling to take such a gamble. Instead, on 20 September, he attempted to break through the French lines at Valmy. The resulting battle was effectively an artillery duel, in which the experienced former Royalists in the French artillery proved that they were still effective, and the French infantry help its ground. Brunwick ordered two infantry attacks, but cancelled both of them well before they could threaten the French lines. After this one setback Brunswick was unwilling to attack again, and after a ten day long standoff he marched his army back to Germany. The battle of Valmy was one of the key battles of European history, and probably saved the French revolution from a quick defeat.
Elsewhere a French army under General Custine took Frankfurt, but he was now dangerously isolated. When Brunswick sent troops east from Coblenz, up the Lahn, and south towards Bingen on the Rhine in early December Custine was forced to abandon Frankfurt.
In 1793 Brunswick besieged Custine in Mainz (14 April-23 July 1793), but it took the Allies two months to move south after the fall of Mainz. The Prussians, under Brunswick, advanced on the Allied right (western) flank, and ended up at Kaiserslautern, before moving slowly to Pirmasens, where they dominated one of the passes across the Vosges.
The French made the next move, sending 12,000 troops to attack Brunswick's base at Pirmasens. The resulting battle of Pirmasens (14 September 1793) was a Prussian victory - the French attack was detected early, and the French were defeated as they attempted to advance up three separate valleys towards the town.
This was followed by a limited Allied invasion of Alsace. The Austrians broke through the Lines of Wissembourg (13 October 1793), close to the Rhine, and advanced south to threaten Strasbourg. The Prussians were less active, and remained on the Saar. The two allied armies were soon separated by the northern end of the Vosges mountains.
By mid-November Brunswick had moved to Blieskastel, from where he attempted to capture the fortress of Bitche, which commanded one of the routes across the Vosges. On 17 November the French under General Hoche counterattacks, but their attack failed. Despite the French failure, Brunswick decided to retreat to Kaiserslautern. A French attack on Kaiserslautern (28-30 November 1793) failed. The French attempted to attack in three columns, and their attacks were badly coordinated. By the time the French managed to get all three columns into the battle, Brunswick had received reinforcements.
Further to the east the French had more success, and were able to storm the Lines of Wissembourg (26 December 1793). This forced the Austrians to retreat back across the Rhine, which in turn left Brunswick dangerously isolated. His response was to retreat back to Mainz and go into winter quarters.
By the time the fighting resumed in the spring of 1794 Brunswick had resigned because of interference from the King, who had left the Rhine front to concentrate on affairs in Poland. Brunswick was replaced by Marshal Richard von Mollendorf.
In 1806 Queen Louisa of Prussia convinced the Duke to come out of retirement and take command of the Prussian army (War of the Fourth Coalition). Just as in 1792 the King of Prussia, by now Frederick William III, accompanied the army, but he was less forceful than his predecessors, and was unable to impose a single strategy on his two commanders (Brunswick and Hohenlohe). After invading Saxony, the Prussians paused to decide what to do. Brunswick wanted to attack to the south-west, Hohenlohe to advances south (the French still had large parts of their army on Austrian territory, where they had stopped at the end of the War of the Third Coalition). Eventually bits of both plans were adopted, and the Prussian army was split in two.
The Prussians had forgotten to take Napoleon into account. His offensive began on 8 October. Two days later, on 10 October, Prince Louis Ferdinand of Prussia, commander of Hohenlohe's advance guard, was killed at the battle of Saalfield (10 October 1806), a defeat and a death that dented Prussian morale. On the same day the Prussians decided to concentrate at Weimar. Brunswick was to lead the main army there, while Hohenlohe took the flank guard to Jena. This plan confused Napoleon, who had expected a more logical attempt to protect Leipzig. This began a short period of confusion, which ended with the twin battle of Auerstädt and Jena (14 October 1806). While Napoleon defeated Hohenlohe at Jena, Marshal Davout inflicted a humiliating defeat on Brunswick and the main Prussian army. Brünswick himself was mortally wounded early in the battle when a missile hit his left eye, leaving the King in rather ineffective command of the army.
After his death his son Frederick William succeeded to the Duchy, but in the following year the French seized his lands to create the new Kingdom of Westphalia. The new Duke became one of the most implacable enemies of the French, and raised a private army which fought for the Austrians and then the British. He became know as the 'Black Duke', and was killed at the battle of Quartre Bras.