The battle of Langensalza (27 June 1866) was the only significant Prussian setback during the campaign against their German enemies during the Austro-Prussian War of 1866, but despite their victory the Hanoverians were forced to surrender two days later.
At the start of the war the Prussians allocated three divisions to the fight against their German opponents. This force, under General von Falckenstein, would have been outnumbered if all of their German opponents had been able to unite, but the three main German forces were just as badly divided as the Prussians. In the south were the Bavarians and the 8th Federal Corps, while in the north was the army of the kingdom of Hanover, under General von Arentschild, but with King George V of Hanover present.
At the start of the war the Hanoverian army concentrated at Gottingen, just over 50 miles to the south of the city of Hanover. They found themselves faced by all three of Falckenstein's divisions. The 13th Division was nearest, and it occupied Hanover on 17 June. Manteuffel's division was approaching from Holstein in the north. Finally Beyer's Division was approaching from the west, and by 19 June was at Cassel. This meant that the Hanoverians had to abandon their first plan, which was to advance through Cassel on their way towards Frankfurt and a union with the Bavarian and Federal forces. Instead they chose to head south, towards Eisenach. As the Hanoverians moved south, Falckenstein ordered his units to concentrate on Göttingen. This gave the Hanoverians time to move south, and by 22-23 June they had reached Langensalza, three quarters of the way to their target. At this point their movement rather ground to a halt. The Prussians had very few troops in front of them - only small detachments at Eisenach and Gotha, and Falckenstein repeatedly ignored orders to use the rail network to reinforce both places.
On 24 June the Hanoverians attacked towards Eisenach. They were probably on the brink of success when news reached the Hanoverian commander on the spot, General Bülow, that peace talks were proceeding well and hostilities should be avoided. A truce was agreed, to last until the following morning. Finally Falckenstein realised the urgency of the situation, and by the time the truce expired there were ten Prussian battalions at Eisenach. A detachment of five battles from Manteuffel's division, commanded by General Flies, was on its way to Gotha, where it arrived on the afternoon of 25 June.
The peace negotiations continued on, hampered by interventions from Moltke in Berlin, often based on out-of-date or incorrect information. On the night of 25-26 June a Hanoverian foraging part was detected at Mühlhausen. This was misreported as a major Hanoverian force, and Moltke assumed that the Hanoverians were retreating north. Falckenstein was sent a direct order to attack the Hanoverians, but he decided that this was based on false information and ignored it. When no attack came on 26 June that order was repeated. Once against Falckenstein decided to ignore it, but General Flies at Gotha didn't know this. He decided to obey the order, and on 27 June advanced north from Gotha towards the Hanoverian position at Langensalza where he expected to find no more than a rearguard of a retreating army.
The Hanoverians were in a strong defensive position on the north bank of the River Unstrut. They had outposts on the south bank of the river, including in Langensalza. Their main force was spread out between the villages of Thamsbrück, Merxleben and Nagelstadt, all on the north bank of the river, and all with a river bridge. The main bridge was in the centre of the line, at Merxleben. Arentschild put Bülow's brigade on his right (at Thamsbrück), Vaux's brigade in the centre (at Merxleben) and Bothmer's brigade on his left (towards Nagelstadt). Knesebeck's brigade was in reserve. The Hanoverians had around 19,000 men available.
The Prussians had around 9,000 men available for the attack. General Flies ordered his men to occupy Langensalza, and the Jüdenhugel, a hill just to the north-east of the town. This gave them a good position for their artillery, overlooking the Hanoverian guns on lower hills north of the river, and the Prussians began a punishing artillery bombardment. The Hanoverians responded by moving Knesebeck's brigade to the front. At this crucial moment Flies was struck down by heat stroke. He was unconscious for the next hour, leaving his force without direction.
The same wasn't true on the Hanoverian side. Arentschild realised that he had a chance to defeat the outnumbered Prussians, and ordered a general attack across the river. The attack began on the Hanoverian right, where Bülow and Knesebeck were able to cross the river and push back the Prussian left. However on the Hanoverian left General Bothmer's first attempt to cross the river was repulsed. He refused to attempt to cross a second time.
The final Hanoverian attack came in the centre. Vaux's brigade, supported by two battalions sent by Knesebeck, pushed across the river and captured Kallenberg's Mill, near the southern end of the bridge.
By this point Flies had regained consciousness, and he decided to order a retreat. The Prussians ended the day back at Warza, just to the north of their starting point at Gotha. The Prussians lost 170 dead, 643 wounded, 33 missing and according to the Hanoverians 907 prisoners, a total of 1,800 casualties. The Hanoverians lost 1,429 killed and wounded, a reflection of their role as attackers, and the power of the Prussia needle gun.
The Hanoverian victory at Langensalza didn't change the overall situation. The Prussians were now concentrating against them, and most of Falckenstein's men were now blocking the Hanoverian's road south to their allies. On the day after the battle the Hanoverian officers told the king that the army was surrounded, short of food and ammunition and in need of rest, and advising him to surrender. Initial negotiations were with Falckenstein, but on 29 June Manteuffel arrived with better terms sent from Berlin. The Hanoverians accepted these terms, and on 29 June signed the capitulation. After the war King George V of Hanover abdicated and his kingdom became part of Prussia.
With the Hanoverians out of the fight Falckenstein was free to move south to deal with the Bavarians and the 8th Federal Corps.