The battle of Arnay-le-Duc (25 June 1570) was a Huguenot victory that forced Charles IX of France to agree to a peace treaty that ended the Third War of Religion.
In the autumn of 1569 the advantage in the war had shifted to the Catholic and Royal side. Colingy had besieged Poitiers, but the siege had been lifted and the Huguenots had been defeated at Moncontour (3 October 1569). Coligny had escaped from this disaster and moved to the south, where he was able to raise a new army, and also to unit with the army of Gabriel de Lorges, comte de Montgomery, the victor of a brief campaign that had seen the Protestants regain control of Béarn.
Although the Huguenots were secure in the south-west, Coligny believed that the only way to secure peace would be to threaten the Royal court in the north. Accordingly, early in 1570 he began to advance up the Rhone valley. By late May the Huguenots had reached St. Etienne, then moved to St. Rambert, on the Loire, where Coligny fell seriously ill. By this point peace negotiations had begin, but while Coligny was ill they were suspended. After Coligny's recovery the army resumed its advance, turning a little away from the Loire and instead heading towards Arnay-le-Duc, on the road from Chalon to Paris.
The nearest Royal army was that of Marshal Cossé, who had been ordered to take La Charité, the main Huguenot stronghold on the Loire. As Coligny moved north, Cossé was ordered to move east to intercept him. The two armies met just to the north of Arnay-le-Duc.
Cossé had around 13,000 men, with 4,000 Swiss and 6,000 French infantry, 3,000 cavalry and 12 cannon. Coligny's force was much smaller. He had around 2,500 arquebusiers and 2,000 cavalry, split roughly equally between well-equipped French troops and more poorly equipped Germans. Coligny was also accompanied by the young Henry of Navarre.
Coligny did have the advantage of terrain. The two armies were posted on rolling hills separated by a stream. At one end of the battlefield were some ponds, and there were two mills on the stream, both of which were held by the Huguenots.
Cossé began the battle by attempting to force his way across the stream, but a series of assaults were turned back by a combination of fire from the arquebusiers and cavalry charges. This stage of the battle lasted for most of the day. Cossé then attempted to outflank Coligny's position and to attack the ponds, but was again repulsed. At the end of the day Cossé withdrew for the night.
On the next morning the Royal army reappeared, but Cossé was unwilling to attack, and Coligny had no reason to. After a brief artillery bombardment the Royal army returned to its camp.
In the aftermath of the battle Coligny slipped away to the west, reaching safety at La Charité. This directly threatened Paris, and played a part in convincing Charles IX to finally agree to many of the Huguenot demands. On 14 July a ten-day long truce was agreed, and this was soon followed by the Peace of St.-Germain, which ended the Third War of Religion.