The Battle of Wolfenbüttel took place on June 29, 1641 near the town of Wolfenbüttel in what is now eastern Germany, as part of the Thirty Years' War. Swedish forces led by Carl Gustaf Wrangel and Hans Christoff von Königsmarck and Bernardines led by French general Jean-Baptiste Budes, Comte de Guébriant withstood a fierce assault by Imperial forces led by Archduke Leopold Wilhelm of Austria, forcing the Imperials to retreat. With a total of around 48,000 men fighting on both sides, it was one of the largest battles of the entire war.
Wolfenbüttel, occupied by Imperial forces, was a strategically insignificant town, but it held great value to the Swedes' Lüneburgian allies. A Lüneburgian army under Hans Caspar von Klitzing had blockaded the Imperial garrison under Johann Ernst, Baron von Ruischenberg since the previous autumn, but its 7,000-man force had been too small to reduce the town. Facing growing uncertainty in the wake of the death of General Johan Banér and mutinous troops following a year of inaction and failure, the Swedes needed to do something to ensure the loyalty of the Lüneburgians and prove to other German Protestant allies that they were still a reliable partner. Thus, they decided to assist in the Lüneburgian siege of Wolfenbüttel. Hearing word of the Swedish advance, an Imperial army under Leopold Wilhelm and Ottavio Piccolomini raced to meet them.
The race was essentially a draw. The Swedes under Wrangel and their Bernardine allies led by Guébriant joined Klitzing's troops on 28 June, and the Imperial army arrived just two hours later. The Swedes numbered 13,000 men - 7,000 infantry and 6,000 cavalry, while the Bernardines numbered 4,000 infantry and 2,000 cavalry and the Lüneburgians numbered 7,000 men in total, for a combined force of around 26,000 men. A carriage draped in black bore the body of the recently deceased General Banér in their midst. General Meanwhile, the Imperials had about 22,000 men all told.
The besiegers had created a fortified camp that was too hard to take by frontal assault, so Leopold Wilhelm ordered flanking attacks on both sides. Piccolomini's cavalry attacked the Swedish left, but was driven back with light casualties. Meanwhile, the Bavarians under Joachim Christian von Wahl attacked the Swedish right through a woods containing a small redoubt manned by Banér's legendary "Old Blue" Swedish regiment. The Bavarians took heavy casualties storming the woods but finally succeeded in taking the redoubt from the "Old Blue," only to be driven back by a cavalry counterattack dispatched by Guébriant and Klitzing from the main camp. At this point, Leopold Wilhelm decided to withdraw, having taken around 3,000 casualties in total, although the Swedes had also paid a heavy price for their "victory," having lost 2,000 men, with the heaviest toll coming out of the most experienced and veteran troops of the "Old Blue" regiment.
The Imperial army withdrew to the south, but the Swedes were unable to pursue, as their mutinous rank-and-file demanded more pay and refused to budge until it materialized. The rest of the summer campaign season was thereby wasted. Meanwhile the Imperial army suffered logistical problems as their demoralized and underpaid soldiers deserted in droves and sold their weapons for money. The war would remain a stalemate until the arrival of from Sweden of Banér's replacement, the brilliant general Lennart Torstensson, the following year. Torstensson not only brought money from Sweden to pay the troops, but also instilled brutal discipline to prevent further mutiny, and was thereby able to get the Swedes back to their winning ways of the past.