Although Dutch law did not forbid a Catholic to reign over the Netherlands, Protestant succession was traditional, born out of the 16th-century Eighty Years' War with Spain and the assassination of William of Orange by a supporter of Philip II of Spain who believed William had betrayed both the Spanish monarch and the Catholic Church. By the middle of the 20th century religious attitudes had begun to change, but only very slowly. While members of the Roman Catholic Church accounted for approximately 34 percent of the Dutch population, and Catholic political parties had been in coalition governments since 1918, the high fertility rate of the Catholics was a matter of some concern for all non-Catholics.