On July 31, 1914, the regiment was assigned to the Sordet cavalry corps, brigaded with the 21st Dragoons in General Lastour's 5th Division. In August, it participated in the raid on Belgium which led to the siege of Liège, then Neufchâteau, Fleurus and Orbais, and then was back in France in Maubeuge on August 21. From August 23 to September 4, the regiment covered the retreat of the French Army after the defeat at the Battle of Charleroi. The regiment was near Versailles on September 5. In the First Battle of the Marne, from September 5 to 14, the 5th Dragoons were engaged in Betz, Nanteuil, Margny, Rosière and Senlis. Following these successes, the regiment received the honour of adding "L'Ourcq 1914" on its banner.