Field Marshal August Wilhelm Anton, Graf Neithardt von Gneisenau (1760-1831) was one of the main Prussian military reformers after the disasters of 1806, but is most famous for his role as Blücher's chief of staff in the campaigns in 1813, 1814 and 1815.
Gneisenau was born as August Wilhelm Anton Neithardt on 27 October 1760 at Schilda, the son of an officer in the Saxon army. He was raised in relative poverty, but was still able to spend two years at Erfurt University. He then began a military career, first in the Austrian army, where he served with the cavalry (1778-1780) and then in the service of the Margrave of Anspach-Bayreuth (1782-83). The Margrave hired his army out to the British, and so Gneisenau fought in the War of American Independence.
In 1786 Gneisenau entered the Prussian service. He spent most of his time on garrison duty in Silesia, where he was able to study military theory in some details.
In 1806 Prussia declared war against France (War of the Fourth Coalition), but then suffered a humiliating series of defeats. Gneisenau served as a staff office at Jena, where his courage under fire gained him notice. He then took part in the famous siege of Colberg (Kolberg), which held out until the end of the war, and it was this that really made his name,
After the disasters of 1806 King Frederick William III appointed General Scharnhorst as head of a military reform commission. Gneisenau was appointed chief of engineers and worked with Scharnhorst on his reforms. Amongst their achievements were the creation of flexible mixed arms brigades, an improvement in the structure and role of the General Staff, preparations for an increase in the size of the army from the 42,000 allowed by Napoleon and a relaxation of some of the more inflexible rules surrounding military service (see Scharnhorst for more details).
Gneisenau was forced into retirement after the political leader Heinrich Friedrich Karl Stein was forced out of power early in 1809. During his time away from Prussia he travelled to Russia, Sweden and England. He then returned to Prussia, where he played a major role in the 'patriotic movement'. He was one of a number of Prussian officers to resign after Frederick William agreed to provide a corps for the French invasion of Russia in 1812.
After the resumption of the war with France in 1813 Blücher was appointed as commander of the Prussian field armies with Scharnhorst as his chief of staff. Scharnhorst was wounded at the battle of Lützen (2 May 1813) and then sent to Austria to try and convince the Emperor Francis to join the war. While there Scharnhorst died after his wound became infected.During the autumn campaign of 1813 Gneisenau replaced Scharnhorst as Blücher's chief of staff, forming one of the greatest partnerships in military history. This partnership lasted throughout the campaigns of 1814 and 1815. Wellington gave a good explanation for its success - stating that Gneisenau was 'very deep in strategy … in tactics … not so much skilled. But Blücher was the reverse - he knew nothing of plans of campaign, but well understood a field of battle'. Over the next two years Gneisenau providing the planning and organisation, while Blücher proved the aggressive attitude, battlefield skills and willingness to lead from the front if needed.
Gneisenau helped control the Allied advance on Paris in 1814, although his influence can perhaps be overstated. Blücher had a tendency to led his army get too widely spread out, and Gneisenau was unable to rein this in enough to avoid a series of defeats during the 'Six Day's Campaign' (Champaubert, Montmirial, Château-Thierry and Vauchamps).
Early in the campaign Napoleon attempted to prevent the main Allied armies from uniting. His efforts failed, as Blücher was moving rather more quickly than expected. Napoleon did win a minor victory at Brienne (29 January 1814), but it might have been much more significant - Blücher and Gneisenau were almost captured in the castle at Brienne, leaving by one gate as the French entered through another.
By early March the Prussians were back on the Marne, with Napoleon pursuing from the south. The Prussians ended up making a stand at Laon (9-10 March 1814), after successfully joining up with reinforcements coming from the north. On the first day of the battle Blücher inflicted a heavy defeat on Marmont's isolated corps, and he issued orders for an attack on the following day that might well have ended the war then and there.
Unfortunately for the Allies Blücher then fell ill with an inflammation of the eyes and Gneisenau took over. He wasn't very experienced as a field officer, and decided to cancel Blücher's planned attacks. This caused a short crisis in the Allied command - General Kleist briefly resigned his command in the belief that Gneisenau was hiding Blücher's death, while the next most senior officer, General Langeron (a French émigré in Russian service) refused to take command. As a result Napoleon was able to withdraw with most of his main army intact.
After Napoleon returned from exile in 1814 Gneisenau was once again appointed as Blücher's chief of staff, although this came over the heads of several more senior officers.
Gneisenau didn't entirely trust Wellington, and believed that the Prussians had been let down at Ligny, after believing that Wellington had broken a promise to come to the aid of the Prussians (Wellington had promised to come if he wasn't attacked, but then had to fight off an unexpected French attack at Quartre Bras).
On the night of 16-17 June this almost led to disaster. Blücher was still missing, and Gneisenau wanted to withdraw towards Liège to reorganise. This would have opened a gap between the two Allied armies, and given Napoleon the chance to defeat Wellington without any Prussian intervention. Luckily for the Allies the Prussians had already retreated past the best road to Liège, and so the decision was made to move north instead. Wavre was chosen as the target as everyone could find it on their maps. A couple of hours later Blücher finally rejoined his headquarters, and a council of war was called. Gneisenau wanted to move to Liège via Wavre, but Blücher, supported by his quartermaster-general Grölman, insisted on supporting Wellington. On 17 June Napoleon wasted the morning, and then made the fatal decision to split his army, sending Grouchy with 33,000 men to pursue the Prussians. The rest of his army moved towards Wellington.
18 June saw the battles of Wavre and Waterloo. At Wavre one Prussian corps help up Grouchy all day. At Waterloo Wellington was able to hold on until the Prussians arrived on his left flank, and was then able to launch an attack that forced the French into retreat. If Gneisenau had got his way on the night of 16-17 June the battle may well have ended as a French victory (Wellington didn't believe this was the case, although he was also willing to give the Prussian credit for their role in the nature of the victory). Gneisenau took part in the advance to Waterloo. The relationship between Blücher and Gneisenau was clearly one between almost equals, rather than a more normal relationship between senior and junior officers - after the council of war Blücher said that 'Gneisenau had given way'. His initial plans have resulted in Gneisenau getting a rather bad press in many British histories of the campaign, although probably unfairly.
After the end of the war the more liberal reforms went out of favour at the Prussian court. Gneisenau retired on health and political grounds, but returned as governor of Berlin in 1818. He was promoted to Field Marshal in 1825. In 1831 he was given command of an Army of Observation on the Polish Frontier (with Clausewitz as his chief of staff), to watch the Russians as they dealt with an uprising. Both men died of cholera during this duty, Gneisenau on 24 August 1831 and Clausewitz later in the year.