From 1618, Europe was convulsed by the religious conflict known as the Thirty Years' War. Despite the peace-loving inclinations of King James I, England became involved in the early phase of the war through royal family connections.
In 1612, James' eldest daughter Elizabeth married Frederick V, the youthful Elector of the Palatinate, which was the leading state in the German Protestant Union. In 1618, the Protestant princes chose Frederick and Elizabeth to be King and Queen of Bohemia in defiance of the claims of the Hapsburg Emperor, Ferdinand II. Imperial troops invaded Bohemia and defeated Frederick at the battle of the White Mountain in 1620, while Spanish and Bavarian forces invaded the Palatinate itself. Frederick and Elizabeth were driven into exile in Holland. English Protestants demanded military intervention to liberate the Palatinate and restore the popular Princess Elizabeth — known as the "Queen of Hearts". A force of English volunteers commanded by Sir Horace Vere rode gallantly to Elizabeth's rescue, but King James realised that full-scale military intervention on behalf of his daughter and son-in-law was too costly.
Instead he sought a diplomatic solution by proposing that Elizabeth's brother Charles, Prince of Wales (later Charles I), should marry the Infanta Maria, sister of King Philip IV of Spain. James hoped that Charles' marriage to a Hapsburg would bring family pressure on Ferdinand to restore Frederick and Elizabeth as well as giving Britain a powerful European ally.
Early in 1623 Prince Charles, accompanied by the Duke of Buckingham, embarked upon an ill-advised journey to Madrid to court the Infanta in person. A papal dispensation was required before the Infanta could marry a Protestant prince. Pope Gregory XV was in favour of the marriage but he died while the negotiations were in progress, after which they broke down in an atmosphere of hostility and mistrust.
Charles and Buckingham returned to England in October 1623, determined to avenge their humiliation by making war on Spain. Against King James' better judgement, they persuaded the Parliament of 1624 to vote funds for war. An army was raised under the command of the mercenary Count Mansfeld and sent to regain the Palatinate. Mansfeld's expedition failed, but the English navy was strengthened in preparation for further campaigns against Spain. Although James would not declare war, Charles and Buckingham pursued their plan, and arranged a marriage alliance with Spain's enemy France.
King James died in March 1625 and Charles inherited the throne. Encouraged by the Duke of Buckingham, Charles sent a naval expedition against Spain that was intended to draw Spanish resources away from the Palatinate, but the English attack on Cádiz was a fiasco. After Buckingham's assassination in 1628, King Charles pursued a more peaceful foreign policy. Despite his best intentions, he never sent any effective military help to restore Frederick and Elizabeth. Frederick died in 1632. Elizabeth remained in exile until the Peace of Westphalia brought the Thirty Years' War to an end in 1648 and her eldest son Charles Louis was restored to the Palatinate.