By the 1580's, Queen Elizabeth's arrest and long imprisonment of Mary, Queen of Scots, created a great deal of resentment against Elizabeth and the English in Catholic countries, most especially in Spain. When Elizabeth had Mary executed in 1587, Catholic priests, mostly Jesuits, coming into England from France had created sufficient unrest among English Catholics so that Elizabeth's supporters began to worry about her safety. In 1585, in fact, Protestant leaders created an association specifically designed to uncover plots against Elizabeth and to arrest and execute any Catholic implicated in a plot. Mary's execution in 1587 was, at least for the Spanish, the point at which they determined to launch an attack on England that would forever destroy a Protestant country that was, for many decades, interfering with their control of the New World. From a political and strategic viewpoint, the attack on England, known now as The Spanish Armada, was the most significant event in the last decades of Elizabeth's reign.
In 1588, after years of collecting information on English sea power, the Spanish, under the Duke of Medina-Sidonia, an experienced admiral who, although he had serious doubts about a massive single attack on the English, launched the Armada from Cadiz. The Armada, which had been assembled using Spanish Navy and civilian ships, consisted of over 130 ships, with about 2,500 guns, and approximately 30,000 sailors and soldiers. They expected to take aboard about 15,000 troops in the Netherlands, and, with the 20,000 troops already aboard the Armada ships, land on the south coast of England and destroy the English. After several engagements with the English between July 21 to about July 28, 1588, which essentially resulted in a draw, Medina-Sidonia, hoping to meet and load the Spanish troops in the Netherlands, anchored of Calais in France to wait for the troops to arrive. By that point, the English were able to concentrate their fleet off Calais.
The main action took place off Calais on July 29, 1588, and cost the Spanish several major warships and many troops. Medina-Sidonia then took the fleet to Gravelines, off the coast of the Netherlands, in hopes of meeting the Spanish troops in their transports and ready to sail to England's south coast. Neither the troops nor their transports were there, and the Armada began a desperate fight with the English for almost 8 hours, which resulted in a complete defeat of the Armada. The remaining ships attempted to sail back to Spain but hit terrible storms off the western coast of Ireland, which destroyed many of the remaining ships, and only about half the Armada made it back to Spain by October, 1588.
The defeat of the Armada was the high-water mark of Elizabeth's last years. Even though the English continued to fight the Spanish whenever possible, circumnavigated the globe, and started the East India Company, which eventually created an empire, Elizabeth was out of money. A rebellion in Ireland further drained the English treasury and Elizabeth's power, and Elizabeth's death on March 24, 1603, ended what most historians believe was the most productive period in English history to that point.