Elizabeth, however, did not feel entirely safe upon her throne. She had a rival and a very dangerous one. Mary, of the house of Stuart, daughter of a French duchess and a Scottish father, widow of king Francis II of France and daughter-in-law of Catherine of Medici (who had organised the murders of Saint Bartholomew's night), was the mother of a little boy who was afterwards to become the first Stuart king of England. She was an ardent Catholic and a willing friend to those who were the enemies of Elizabeth. Her own lack of political ability and the violent methods which she employed to punish her Calvinistic subjects, caused a revolution in Scotland and forced Mary to take refuge on English territory. For eighteen years she remained in England, plotting forever and a day against the woman who had given her shelter and who was at last obliged to follow the advice of her trusted councilors "to cutte off the Scottish Queen's heade." The head was duly "cutte off" in the year 1587 and caused a war with Spain. But the combined navies of England and Holland defeated Philip's Invincible Armada, as we have already seen, and the blow which had been meant to destroy the power of the two great anti-Catholic leaders was turned into a profitable business adventure. For now at last, after many years of hesitation, the English as well as the Dutch thought it their good right to invade the Indies and America and avenge the ills which their Protestant brethren had suffered at the hands of the Spaniards. The English had been among the earliest successors of Columbus. British ships, commanded by the Venetian pilot Giovanni Caboto (or Cabot), had been the first to discover and explore the northern American continent in 1496. Labrador and Newfoundland were of little importance as a possible colony. But the banks of Newfoundland offered a rich reward to the English fishing fleet. A year later, in 1497, the same Cabot had explored the coast of Florida.