The Battle of Valmy, also known as the Cannonade of Valmy, was the first major victory by the army of France during the Revolutionary Wars that followed the French Revolution. The battle took place on 20 September 1792 as Prussian troops commanded by the Duke of Brunswick attempted to march on Paris. Generals François Kellermann and Charles Dumouriez stopped the advance near the northern village of Valmy in Champagne-Ardenne.

In this early part of the Revolutionary Wars—known as the War of the First Coalition—the new French government was in almost every way unproven, and thus the small, localized victory at Valmy became a huge psychological victory for the Revolution at large. The outcome was thoroughly unexpected by contemporary observers—a vindication for the French revolutionaries and a stunning defeat for the vaunted Prussian army. The victory emboldened the newly assembled National Convention to formally declare the end of monarchy in France and to establish the French Republic. Valmy permitted the development of the Revolution and all its resultant ripple effects, and for that it is regarded by historians as one of the most significant battles in history.

As the French Revolution continued, the monarchies of Europe became concerned that revolutionary fervor would spread to their countries. The War of the First Coalition was an effort to stop the revolution, or at least contain it to France. King Frederick William II of Prussia had the support of Great Britain and the Austrian Empire to send the Duke of Brunswick towards Paris with a large army. In the war's early encounters of mid-1792, French troops did not distinguish themselves, and enemy forces advanced dangerously deep into France intending to pacify the country, restore the traditional monarchy, and end the Revolution. The French commander Charles Dumouriez, meanwhile, had been marching his army northeast to attack the Austrian Netherlands, but this plan was abandoned because of the more immediate threat to Paris. A second army under General François Kellermann was ordered to link up with him in a mutual defense.

Just over half of the French infantry were regulars of the old Royal Army, as were nearly all of the cavalry and, most importantly, the artillery, which were widely regarded as the best in Europe at the time. These veterans provided a professional core to steady the enthusiastic volunteer battalions. Combined, Dumouriez' Army of the North and Kellermann's Army of the Centre totalled approximately 54,000 troops. Heading towards them was Brunswick's coalition army of about 84,000, drilled Prussian and Austrian troops augmented by large complements of Hessians and emigres of the French royalist Army of Condé.

The invading army handily captured Longwy on 23 August and Verdun on 2 September, then moved on toward Paris through the defiles of the Forest of Argonne. In response, Dumouriez halted his advance to the Netherlands and reversed course, approaching the enemy army from its rear. From Metz, Kellermann moved to his assistance, joining him at the village of Sainte-Menehould on 19 September. The French forces were now east of the Prussians, behind their lines. Theoretically the Prussians could have marched straight towards Paris unopposed, but this course was never seriously considered: the threat to their lines of supply and communication was too great to be ignored. The unfavorable situation was compounded by bad weather and an alarming increase in sickness among the troops. With few other options available, Brunswick turned back and prepared to do battle.

The troops trudged laboriously through a heavy downpour—"rain as of the days of Noah", in the words of Thomas Carlyle. Brunswick headed through the northern woods believing he could cut off Dumouriez. At the moment when the Prussian manœuvre was nearly completed, Kellermann advanced his left wing and took up a position on the slopes between Sainte-Menehould and Valmy. He centered his command around an old windmill, which he quickly razed to prevent enemy artillery spotters from using it as a sighting location. His veteran artillerists were well-placed upon its accommodating ridge to begin the so-called "Cannonade of Valmy". Brunswick moved toward them with about 34,000 of his troops. As they emerged from the woods, a long-range gunnery duel ensued and the French batteries proved superior. It was very unusual when the armies faced one another. The Prussians had their backs towards Paris, while the French faced toward Germany rather than barring the road to Châlons. This configuration contributed to a strange beginning to an already unconventional battle. A change in Brunswick's line of march brought the Prussian main force into direct contact with Kellermann rather than Dumouriez's volunteers. Kellermann deployed his army in a broad arc, with his left along Châlons road, his center on a hill west of Valmy, and his right anchored on Mount Yron. The Prussian infantry made a cautious, and fruitless, effort to advance under fire across the open ground.

Early French artillery fire was affected by fog and rain, but a battery positioned near the inn of La Lune successfully stopped the Prussian cavalry from advancing. Other French artillery units joined Kellermann on the heights. The French infantry advanced only a few hundred feet, while the Prussian infantry advanced roughly 200 yards, repeatedly halted by heavy artillery fire. During the prolonged cannonade, Prussian fire detonated three French ammunition wagons. Despite this setback, Kellermann steadied his infantry by bringing up artillery.

As the Prussians wavered, a pivotal moment was reached when Kellermann raised his hat and made his famous cry of "Vive la Nation". The cry was repeated again and again by all the French army, and had a crushing effect upon Prussian morale. The French troops sang "La Marseillaise" and "Ça Ira", and a cheer went up from the French line. Although several French armies were nearby, Kellermann carried out most of the fighting himself. Dumouriez's army stayed southeast of Sainte-Menehould, Beurononville's troops remained behind Kellermann, and Dillion's forces at Les Islettes stopped Hohenlohe's troops from reinforcing the Prussians. Because of this, Kellermann fought the Battle of Valmy largely on his own. To the surprise of nearly everyone, Brunswick broke off the action and retired from the field. As the night approached, Brunswick, King Frederick William II, and the Prussian officers met near the inn of La Lune. During this meeting, Brunswick reportedly said, "Hier schlagen wir nicht" ("Here we will not fight [or win]"), showing that the Prussian leadership believed the battle couldn't be continued. The Prussians rounded the French positions at a great distance and commenced a rapid retreat eastward. The two engaged forces had been essentially equal in size, Kellermann with approximately 36,000 troops and 40 cannon, and Brunswick with 34,000 and 54 cannon. Yet by the time Brunswick retreated, casualties had risen no higher than three hundred French and two hundred Prussians. After the battle, Kellermann moved his troops south of the Châlons road, leaving the Prussians a clear path to retreat. Dumouriez, who took command of both French armies after Valmy, negotiated with the Prussians while strengthening his forces. On September 30, Brunswick withdrew toward Germany without being attacked, as French commanders were ordered not to engage.