The Union of Arras (Dutch: Unie van Atrecht, French: Union d'Arras, Spanish: Unión de Arrás) was an alliance between the County of Artois, the County of Hainaut and the city of Douai in the Habsburg Netherlands in early 1579 during the Eighty Years' War.
Dissatisfied with the religious policies of rebel leader Prince of Orange and the States General of the Netherlands, and especially the rise of the radical Calvinist Republic of Ghent since October 1577, they signed a declaration on 6 January 1579 about their intent to offer a vigorous defense of the Roman Catholic religion against what they saw as encroachments by Calvinists in other provinces.
[1] One of the important provisions of the Pacification was that the Calvinists received freedom of religion in Holland and Zeeland, and would be tolerated elsewhere in the Netherlands, but that the other 15 provinces would officially maintain the Catholic Church as the dominant one.
In Flanders and Brabant they even used force to change the government of cities like Ghent, Bruges, and Antwerp to obtain this objective, much to the dismay of Catholic politicians in the southern part of the country.
[3] Don Juan died in October 1578 and was replaced as commander of the Spanish Army of Flanders and royal governor-general by Alexander Farnese, Duke of Parma, who proved an able diplomat, adept at driving wedges between the Catholic nobles in the south and the regime of Orange and the States General.