Elected deputy in 1919 and president of the Army Commission in the legislature, he then took the head of a confessional political movement, the Fédération Nationale Catholique.

For a long time controversial because of a Catholicism that was considered outrageous by his opponents, historians[1] 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.

His career was delayed for the first time when the polemicist Urbain Gohier, in an article in L'Aurore,[4] revealed that he was the descendant of an emigrant, who had fought in the army of the Prince of Condé during the Revolution.

The Chief of Staff, General Delanne, opposed this decision and appointed Castelnau to command the 37th Infantry Regiment in Nancy and then resigned.

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.

Georges Clemenceau, although in favour of the law, immortalised tat antagonism by giving Castelnau nicknames like the "Fighting Friar", which have become legendary.

While the Grand Quartier Général (GQG) claimed that the Germans were in retreat[7] 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.

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).

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.

In Picardy, Castelnau distinguished himself by resisting a German offensive commanded by General Alexander von Kluck in the Roye region.

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.

[9] 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.

After that feat of arms, he was made a Grand Croix de la Legion d'Honneur on 8 October 1915 and, two months later, on 11 December 1915, he was appointed Chief of the General Staff of the French Armies, a position that he held throughout 1916.

After three days of fighting, the French defences were in the process of giving way, and Castelnau went to Verdun and took the crucial decisions that would allow the resistance to take hold.

[13] After six weeks of fighting, he decided to appoint General Robert Nivelle, with Pétain taking command of the Centre Army Group (GAC).

He supervised the plans prepared by the French GHQ for this battle and he took part in the preparatory meetings held with Joffre, Haig and Robertson.

In the spring of 1918, taking advantage of the Russian withdrawal from the conflict after the Bolshevik Revolution, the Germans brought all their forces back to France and Belgium and then launched a series of major offensives that were on the way to making them victorious.

On the other hand, as the Franco-British troops, reinforced by the American contingent, regained the initiative during the summer, he was appointed to prepare a decisive manoeuvre in Lorraine.

Despite the additional losses that would have caused ("I know only too well the bitterness of the tears shed on the graves", he wrote to his family and thought of his three sons, Gerald, Xavier and Hugues, who had been killed in the war), Castelnau believed that the Allies should not have signed the armistice prematurely.

It was undoubtedly his active participation in political life that prompted the government of Aristide Briand and War Minister Louis Barthou to remove him from the new list of Marshals announced on 19 February 1921.

Despite a strong movement of public opinion, as shown by the poll carried out by the daily newspaper Le Journal in favour of his nomination, Castelnau was never made Marshal.

However, faced with the resurgence of an anticlerical policy implemented by the new President of the Council (Prime Minister), Édouard Herriot, he launched the idea of a vast national federation of various Catholic movements.

At its head, he forced the government to abandon its entire anticlerical programme in the face of the large demonstrations that Castelnau organised throughout France.

It was not until the early 21st century that contemporary historians such as René Rémond have corrected that image and described him as a moderate right-wing Republican with social ideas ahead of his time.

During the burial ceremony, the Bishop of Toulouse, Jules-Géraud Saliège, although very handicapped, had himself carried into the church to honour the memory of Castelnau to whom he was very close.

[30]' Field Marshal Haig was very admiring of Castelnau's victory at the Battle of the Trouée de Charmes, which he described as an "enormous victory"[31] In his memoirs, Major-General James Harbord of the American Expeditionary Force stated, "It was General de Castelnau, whom many considered the best French general, but a royalist and a Catholic, and therefore suspect.

[33] Colonel Charles à Court Repington, a war correspondent, reported in The Times after his visit to Verdun the words of General de Castelnau: "Rather than submit to German slavery, the whole French race will perish on the battlefield".

[34] In his tribute to the army for the newspaper L'Écho de Paris on 14 July 1919, Castelnau wrote, "The French infantry triumphed over this infernal outburst of fury and horror that surpassed anything the human imagination could ever conceive".

In him, senile pride when 'he gives his person to France', defeatism, intellectual weakness compete with cowardice [...] The Marshal's government is awful in its mentality.