Philip II of Spain, the most powerful monarch in sixteenth-century Europe and a ferocious empire-builder, was matched against the dauntless queen of England, Elizabeth I, determined to defend her country and thwart Philip's ambitions. Philip had been king of England while married to Elizabeth's half-sister, Bloody Mary Tudor, a devout Catholic. After Mary's untimely death, he courted Elizabeth, the new queen, and proposed marriage to her, hoping to build a permanent alliance between his country and hers and return England to the Catholic fold. Lukewarm to the Spanish alliance and resolute against a counterreformation, Elizabeth declined his proposal. When under her guidance England's maritime power grew to challenge Spain's rule of the sea and threaten its rich commerce, Philip became obsessed with the idea of a conquest of England and the restoration of Catholicism there, by fire and sword. Elizabeth—bold, brilliant, defiantly Protestant—became his worst enemy. In 1586 Philip began assembling the mighty Spanish Armada, and in May 1588 it sailed from Lisbon. With superior seamanship and strategies, Elizabeth's navy defeated and drove off the Spanish fleet. Forced to retreat around the northern coast of Ireland and Scotland, Philip's ships ran into violent storms that wreaked havoc. It was the rivalry's climactic event.