The war broke out in 1618 when the Protestant nobility of Bohemia revolted against their newly elected king, Archduke Ferdinand of Styria, a Habsburg. In 1619, they offered their throne to James' son-in-law, Frederick V, Elector Palatine of the Rhine, a leading Protestant. Frederick accepted the offer before taking advice from James. Archduke Ferdinand was elected as Holy Roman Emperor in 1619. He invaded Bohemia, defeating Protestant forces at the Battle of the White Mountain. The Spanish Habsburgs joined and attacked Frederick's hereditary lands in the Palatinate with forces under their general, Spinola. This was believed to be tactical, as they needed to keep the 'Spanish Road' open so they could move troops easily from their lands in Italy. By April 1621, Frederick and Elizabeth were refugees.