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The Armagnac faction was prominent in French politics and warfare during the Hundred Years' War. It was allied with the supporters of Charles, Duke of Orléans against John the Fearless after Charles' father Louis of Orléans was killed on a Paris street on the orders of the Duke of Burgundy on 23 November 1407.
The Armagnac Faction took its name from Charles' father-in-law, Bernard VII, Count of Armagnac (1360–1418), who guided the young Duke during his teens and provided much of the financing and some of the seasoned Gascon troops that besieged Paris before their defeat at Saint-Cloud.
In 1407, Louis of Orléans was assassinated on the order of John the Fearless. Fearing Burgundian ambitions, the dukes of Berry, Brittany, and Orléans, and the counts of Alençon, Clermont, and Armagnac, formed a league against the duke of Burgundy in 1410. Charles of Orléans, son of the murdered Louis, married Bonne d'Armagnac, daughter of Bernard VII, count of Armagnac. In consequence, his father-in-law became the nominal head of the family. For that reason, Orléanist were called Armagnacs.
Parisian supporters of the nobles adopted the name "Armagnac" in the struggle for control of the city against the Burgundians. It was composed of two elements: the Orleanists and those following the Count who gradually infiltrated the noble opposition. Armagnac became an outspoken adherent of the Orleanist Faction in the Valois Court. His Gascon raiders hired to impose order on Paris wore their white shoulder sash. But Armagnac's brutal tactics made his administration very unpopular among Parisians. In February the citizens asked the exiled-John the Fearless to return to the capital. The following month he presented a long document known as The Justification of the Duke of Burgundy containing proof of the Armagnac schemes of intrigue. Orleans pleaded with the king, but Charles insisted on setting a meeting in Chartres to conjure a reconciliation. Meanwhile, by the end of December 1409, the Burgundians had infiltrated all posts of local officials to take control of city governance. The Armagnacs withdrew altogether from city politics to form the League of Gien, they were joined by disaffected Princes of the Blood: John, Duke of Berry, the youngest brother of King Charles V, Louis, Duke of Anjou, John, Duke of Bourbon, John I, Duke of Alençon, John V, Duke of Brittany, Charles d'Albret, Constable of France, and John, Count of Clermont. These nobles formed the political and military elite of the Armagnac faction. The Burgundians met them at the Peace of Bicetres, an attempted truce designed to iron out their differences. It largely failed because as the Armagnacs laid siege to Paris, a small English force landed at Calais to assist the Burgundian government. In October 1411 they marched towards Paris.
Both parties in the Armagnac-Burgondian civil war sought support from Henry IV, King of England. In May 1412, the Armagnacs suffered a second reverse at the Treaty of Bourges. The Armagnacs offered Henry IV full sovereignty in Gascony in return for an army of 4,000 men. Thomas, Duke of Clarence, a fiery cavalry general, demanded considerable territorial concessions including Normandy in return for aid to Burgundy. Now desperate to save the honour of the Oriflamme the Armagnacs resorted to seeking English arbitration in the internal dispute. At the Treaty of Buzancais the English demanded a punitively large ransom from the Armagnacs. In a series of humiliating encounters their leading general, Dauphin Louis, Duke of Guyenne was outmanoeuvred, defeated and forced into a Treaty of Auxerre.
Later, John the Fearless was sent back to his lands, and Bernard of Armagnac remained in Paris and, some say, in the queen's bed. Burgundy gained control of Paris in 1419. The Count of Armagnac was assassinated in the same year. In the same year, Henry V conquest Normandy. For both parties, it was clear that England was the main threat. There was an attempt at reconciliation between Armagnacs and Burgundians. However, in a meeting on the bridge at Montereau in September 1419, the dauphin's entourage assassinated Jean The Fearless of Burgundy. As consequence, Phillippe the Good allied with England as Henry V advanced without opposition to Paris.
Armagnac faction together with the dauphin established a separate jurisdiction in central and southern France, meanwhile, the Anglo-Burgundian alliance controlled the north, including Paris. Sporadic warfare continued between the Armagnacs and Burgundians for a number of years, although after the Burgundians allied themselves with the English in 1419 and the Armagnacs became interlinked with the cause of Charles VII, the factional rivalry was scarcely distinguishable from the Royal dispute between the French and English monarchies.
The terms remained in use until they were outlawed by Charles VII toward the close of the Hundred Years' War, as part of efforts to heal the factional rift.