The reigns of both Robert II and his successor, Robert III, were marked by a general decline in royal power. When Robert III died in 1406, regents had to rule the country; the monarch, Robert III's son James I, had been taken captive by the English. Having paid a large ransom, James returned to Scotland in 1424; to restore his authority, he used ruthless measures, including the execution of several of his enemies. He was assassinated by a group of nobles. James II continued his father's policies by subduing influential noblemen but he was killed in an accident at the age of thirty, and a council of regents again assumed power. James III was defeated in a battle against rebellious Scottish earls in 1488, leading to another boy-king: James IV. In 1513 James IV launched an invasion of England, attempting to take advantage of the absence of the English King Henry VIII. His forces met with disaster at Flodden Field; the king, many senior noblemen, and hundreds of soldiers were killed. As his son and successor, James V, was an infant, the government was again taken over by regents. James V led another disastrous war with the English in 1542, and his death in the same year left the Crown in the hands of his six-day-old daughter, Mary. Once again, a regency was established. Mary, a Roman Catholic, reigned during a period of great religious upheaval in Scotland. As a result of the efforts of reformers such as John Knox, a Protestant ascendancy was established. Mary caused alarm by marrying her Catholic cousin, Henry Stuart, Lord Darnley, in 1565. After Lord Darnley's assassination in 1567, Mary contracted an even more unpopular marriage with the Earl of Bothwell, who was widely suspected of Darnley's murder. The nobility rebelled against the queen, forcing her to abdicate. She fled to England, and the Crown went to her infant son James VI, who was brought up as a Protestant. Mary was imprisoned and later executed by the English queen Elizabeth I. Personal union and republican phase Elizabeth I's death in 1603 ended Tudor rule in England. Since she had no children, she was succeeded by the Scottish monarch James VI, who was the great-grandson of Henry VIII's older sister and hence Elizabeth's first cousin twice removed. James VI ruled in England as James I after what was known as the "Union of the Crowns".