Noël Édouard, vicomte de Curières de Castelnau (French: [kastɛlno]; 24 December 1851 – 19 March 1944) was a French military officer and Chief of Staff of the French Armed Forces during the First World War. Elected to the Chamber of Deputies in 1919 for Aveyron and president of the Army Committee in the legislature, he then took the head of a confessional political movement, the Fédération Nationale Catholique. During the Second World War, he opposed Marshal Pétain and the Vichy regime and supported the French Resistance. For a long time controversial because of a Catholicism that was considered outrageous by his opponents, historians have moderated that portrait by emphasising his great loyalty to republican institutions and disputed in particular that he could have been reactionary or anti-Semitic.

The son of Michel de Castelnau, mayor of Saint-Affrique, Noël Édouard Marie Joseph de Curières de Castelnau was born in Saint-Affrique, into an aristocratic family of the Rouergue. He was the third of five children. His elder brother, Léonce, was a politician of national importance, the president of the parliamentary group of the Action libérale in the National Assembly. His other brother, Clément, was director of the École des mines de Saint-Étienne.

Ruined by the French Revolution, his family had to share a house in Saint-Affrique with his mother's three uncles, the Abbés Barthe. They would have liked him to be a notary, but he wanted to be a naval officer. For reasons of age, he had to reorient himself towards the Army and became part of the 54th promotion of Saint-Cyr, Promotion du Rhin (1869-1871), from which he graduated as second lieutenant on 14 August 1870. He was appointed to the 31st Infantry Regiment and took part in the Franco-Prussian War of 1870 in the Loire Army.

After the war, he served as a lieutenant and then captain in various regiments before joining the École de Guerre in 1879. Assigned to the Army Staff in Paris in 1893, he headed the 1er bureau in 1897. His career was delayed for the first time when the polemicist Urbain Gohier, in an article in L'Aurore, revealed that he was the descendant of an emigrant, who had fought in the army of the Prince of Condé during the Revolution. In 1900, he was the target of the new Minister of War, General André, who wanted to dismiss him from the army because of his aristocratic origins and his Catholicism. According to André, Castelnau did not have the republican profile that he wanted to impose on the army. The Chief of Staff, General Delanne, opposed this decision and appointed Castelnau to command the 37th Infantry Regiment in Nancy and then resigned. That led to the government being questioned in the French Chamber of Deputies and the French Senate. The minister took his revenge by keeping Castelnau in that post for five years, twice the usual length of time in that type of command. He also ensured that Castelnau was not promoted to general despite his record of service, but the affair of the files would revive his career. General André was forced to resign and a few months later, on 25 March 1906, at the request of Paul Doumer, Castelnau was promoted to General de Brigade. He successively commanded a Brigade at Sedan, then at Soissons. On 21 December 1909, he became a General de Division (equivalent to the English language rank of major general), which put him under the command of General Joseph Joffre for the first time. Joffre commanded the army corps on which the 13th Division of Chaumont depended, which Castelnau inherited. Both men got to know each other, and when Joffre was appointed head of the French Army on 2 July 1911, he insisted on having Castelnau at his side. Castelnau took the title of First Deputy Chief of Staff. His main task was to devise a new plan for mobilising and concentrating the French armies in the event of war, Plan XVII. In 1912, he was confirmed in his position as Chief of Staff and replaced General Dubail. By the decree of 30 October 1913, he was then appointed to the Conseil supérieur de la guerre and do he would take command of 2nd French Army in the event of conflict. In 1913, Castelnau found himself largely exposed to the violent debate that accompanied the Three Years' Law. Indeed, when Plan XVII was drawn up, it quickly became clear that military manpower had to be increased in peacetime. The only way to achieve that was to extend military service by an additional year, but almost two thirds of the radical and socialist deputies were fiercely opposed to the prospect of a three-year service. Led by Jean Jaurès, opposition to the bill quickly took a passionate turn. Castelnau, who was considered to have inspired the text, became the bête noire of the opponents, especially as the text was finally voted on 19 July 1913. Resentment towards Castelnau on the part of the radical-Socialist Party continued until the end of his life. Georges Clemenceau, although in favour of the law, immortalised that antagonism by giving Castelnau nicknames like the "Fighting Friar", which have become legendary.

At the declaration of war, he joined his army being mobilised in Nancy. On 15 August 1914, the five French armies went on the offensive against the Germans, who were in the process of making a large overrun through Belgium. Castelnau faced the army of Prince Rupprecht of Bavaria, which was waiting for him on positions prepared in advance at Morhange. While the Grand Quartier Général (GQG) claimed that the Germans were in retreat and that there were only rearguards in front of him, Castelnau suddenly came up against considerable forces that were strongly supported by heavy artillery.

The Second French Army, composed in particular of XV, XVI and XX Corps and 2nd Reserve Division Group the last was commanded by General Léon Durand), suffered heavy losses and had to withdraw to Nancy. Fortunately, Castelnau succeeded in reforming his army, which he later launched into a flanking manoeuvre that inflicted a heavy defeat on the pursuing Germans: the Battle of the Trouée de Charmes (24-27 August) prevented the French armies retreating towards Paris from being turned to the right and made the First Battle of the Marne. Just as the other armies won the victory of the Marne, Castelnau blocked a new German offensive aimed at Nancy at the Battle of Grand Couronné (4-13 September). That earned him the nickname as the "Saviour of Nancy".

On 18 September 1914, Castelnau was promoted to Grand Officer de la Legion d'Honneur. Joffre then withdrew him from the Lorraine front and entrusted him with the mission of extending the left flank of the French armies to the north of the Oise by trying to outflank the German right wing. That was the beginning of the race to the sea, which Castelnau initiated and led to Arras. The manoeuvre was then pursued until it reached the North Sea coast by the British Expeditionary Corps, the Belgian Army and several French army corps under the command of General Foch. In Picardy, Castelnau distinguished himself by resisting a German offensive commanded by General Alexander von Kluck in the Roye region. After the war, that earned him this appreciation from his former adversary: "The French adversary towards whom our sympathies instinctively went, because of his great military talent and his chivalry, is General de Castelnau. And I would like him to know that".'

From November 1914 onwards, the fighting in Belgium and France took the form of trench warfare. Implementing new tactical principles, notably by launching his infantry under the protection of a rolling artillery barrage, Castelnau won a victory at Le Quesnoy-en Santerre. From early 1915, he advocated adopting a defensive attitude on the French front until he had enough heavy artillery to break through the German defences and, in the meantime, to launch a major offensive in the Balkans. His idea was supported by President Raymond Poincaré and Foreign Minister Aristide Briand but opposed by Joffre and the GQG.