The Siege of Derry formed part of the Williamite War in Ireland between 1688-91 and to fully understand it we need to look at events in Europe and England. Louis XIV of France had renaged on the Treaty of Nantes which had given the Protestant French Huguenots religious freedom, he had  reformed the French army and expanded the borders of France into Alsace and the Principality of Orange in Holland.

In England James ll a devout Roman Catholic and first cousin of Louis XIV, suceeded Charles II to the throne (Charles had converted to Catholicism on hs death bed). This brought a great change of politics and religious preferences. In Ireland the Earl of Tyrconnell Richard Talbot acting as James II Viceroy  began dismissing high ranking Protestants from the Irish army and replacing them with Roman Catholics. He also began ensuring that every garrison in Ireland was loyal to the cause of the Catholic King James.

Then the birth of a son to James ll by his second wife, effectively marked the end of any  XIVProtestant desecendency to the throne through his daughter Mary who was married to William III, Prince of Orange.

This spurred several influential Protestant noblemen to conspire against the king. They sent a letter inviting William of Orange, to take the English throne. This offer was taken up and William sailed to England with an army of 15,000 men. James's daughter and many of the Protestant military leaders defected to William and despite having a superior army James did not face William in battle but fled to France to gain support for his cousin Louis XIV.

You may be detecting by now that this was somewhat of a family affair and in many ways it was a political and religious fallout between powerful relatives, all the antagonists were related to each other in one way or another. William was married to the daughter of James II and through marriage both kings were related to Louis XIV of France.