Queen Mary II of England and of Scotland was the daughter of King James II, and the husband of William of Orange. The English Parliament rose against her father in the Glorious Revolution because of his ambitions of returning England to Catholicism. Parliament invited the firmly protestant Mary to take the throne. However she refused to do so unless her husband was named King as well. Parliament agreed and William and Mary took the throne. Mary had the legitimate claim of succession to the throne, while William had the armies of the Dutch republic to defend it. After winning the campaign against James II, the two monarchs ruled jointly.
On the continent William used England's forces to continue his long series of wars against Louis XIV of France. This kept him out of England for much of his reign. In his absence it was Mary who dealt with the court and parliament of England. She was a expert politician and controlled the system with ease giving her husband a great deal of freedom to wage his wars.
Mary died of smallpox in 1694, however, and William was left to run the kingdom alone.
See also British Monarchs