The tension between Spain and England boiled down to the tensions between two people, Mary and Elizabeth. Elizabeth, the charming, flirtatious, virgin Queen who was subtle and a peaceful protestant, against Mary the conservative and strict catholic. They both knew each other's weaknesses and played on them. Queen Elizabeth knew she could have Mary assassinated at any moment, while Mary knew that Queen Elizabeth's navy in England was no match for the strong indomitable catholic Spanish Armada which would surely ruin England and all Protestantism. .
The Spaniards had obviously a stronger navy than the English did and Spain did believe they could beat anyone especially the English. Spain used this to their advantage intimidating all other navy's and even were given the name the Invincible Spanish Armada. The English knew this and knew that if the Spanish Armada beat them, Phillip II the king of Spain at the time, would obviously force them to never rebel against Spain again, and they would force all of England and most of Europe to become Catholics. Knowing this, Elizabeth attempted to remain peaceful with Phillip II, also with Spain in general, and even being tolerant on the Catholics that resided in her own country. .
However, when Elizabeth began to hear of Mary plotting the Queens murder, she started to question whether being tolerable of the Spaniards and Mary was the right choice. With Spain planning the demise of her country and religion and with Mary planning the demise of her she was sure to die. It was time to stop taking the mellow approach, something had to be done before her future vanished and the future of her country and her main religion Protestantism vanished as well. .
If Spain was to invade England and Queen Elizabeth was executed, Philip II of Spain would then take over. He would force all Protestants to become catholic and all Protestantism would be erased not only in England as well as many other parts of Europe that weren't already Catholic.