In the autumn of 1585 Queen Elizabeth signed the Treaty of Nonsuch with the Estates General of the Low Countries. By pledging to send English troops and supplies to aid the Estates in their rebellion against Philip II of Spain, Elizabeth’s actions were seen as a declaration of war. Such action was incredibly risky given that Philip’s power far outclassed Elizabeth’s. This thesis will answer why it was that Elizabeth elected to enter formal war to aid the Dutch in 1585 given that such a dangerous prospect had been avoided at all costs in the years prior. It is argued here that a crucial factor in the Privy Council’s, and consequently Elizabeth’s, decision to enter a formal war against Spain was the Catholic propaganda campaign of 1584, a factor absent in previous answers to this question. In this campaign exiled Jesuit priests made calls for religious toleration. The Elizabethan regime, increasingly paranoid by the revelation of the Throckmorton Plot and the assassination of William of Orange, interpreted this Catholic campaign as the subtle roots for rebellion. They thus it necessary to enter a formal war in the Low Countries as a means to outsource war which they considered inevitable.