A religious war or a war of religion, sometimes also known as a holy war, is a war which is primarily caused or justified by differences in religion and beliefs. In the modern period, there are frequent debates over the extent to which religious, economic, ethnic or other aspects of a conflict are predominant in a given war. The degree to which a war may be considered religious depends on many underlying questions, such as the definition of religion, the definition of 'religious war', and the applicability of religion to war as opposed to other possible factors. Answers to these questions heavily influence conclusions on how prevalent religious wars have been as opposed to other types of wars.

The War of the Jülich Succession was a war of succession in the United Duchies of Jülich-Cleves-Berg. It lasted between 10 June 1609 and 24 October 1610, resumed in May 1614 and finally ended on 13 October 1614. The first round of the conflict pitted Catholic Archduke Leopold V against the combined forces of the Protestant John Sigismund, Elector of Brandenburg and Wolfgang Wilhelm, Count Palatine of Neuburg, ending in the former's military defeat. The representatives of Brandenburg and Neuburg later entered into a direct conflict after their religious conversion to Calvinism and Catholicism respectively. The conflict was further complicated by the involvement of Spain and the Netherlands making it part of the Eighty Years' War, as well as the European wars of religion. It was finally settled by the Treaty of Xanten, whose provisions favoured Spain.