Phillip II of Spain (15 May 1527-13 September 1598), the son of Charles V, the Holy Roman Emperor, was married to Mary I of England from 1554 until her death in 1558, although he had, in effect, left her before then and when it became clear she would remain childless.
Although it was in the interest of the Habsburgs (of which Charles was emperor) that a Catholic monarch reigned in England, Mary Queen of Scots, with her French upbringing and her betrothment to the heir of the French throne, was anyone but their choice. As David Starkey writes: 'Her succession to England would thus be the fulfilment of the worst Habsburg nightmare: she would fuse both island-kingdoms into a joint Franco-British realm that would control the Channel and rupture sea links between the eastern and western halves of the Habsburgs' European empire.' Once he had succeeded his father, Philip remained friendly towards England until he sent his Armada, in 1588, on an invasion attempt to England. Unsuccessfully as it turned out.
Mary Stuart (8 December 1542 - 8 February 1587) was the daughter of King James V of Scotland. Her father having died soon after her birth, she was crowned Queen of Scotland when only a baby. She married Francis, the heir to the French throne in 1558 and a year later he became Francis II of France, with Mary as his Queen Consort, until his death in 1560. Mary returned to Scotland in 1561 and married Henry Stuart, Lord Darnley. Their marriage lasted until 1565 when, after an explosion at the house in which he was staying, he was found dead in the garden, apparently by strangulation. Her life continued on its eventful path with her marriage, soon after, to James Hepburn, 4th Earl of Bothwell but she was soon to be imprisoned in Loch Leven Castle during an uprising and forced to abdicate her throne in favour of her son, the infant James VI. (He became James I of England after the death of Elizabeth I). She managed to escape imprisonment and fled to England, there to seek protection from her cousin, Elizabeth I. Unfortunately for Mary, because many English Catholics supported her claim over that of Elizabeth's, that queen considered it too dangerous to her own position to allow Mary the freedom to gather a threatening force around her. Mary had also always claimed the throne of England as hers by right of descent and legitimacy. Elizabeth had Mary imprisoned in various locations for 19 years, until Mary was eventually tried and found guilty of plotting against Elizabeth. Reluctantly, for Mary was both Elizabeth's cousin and an anointed queen, Elizabeth signed Mary's death warrant. Mary was beheaded at Fotheringhay Castle on 8 February 1587.