Battle of Breitenfeld
On 4 July 1630, Gustavus Adolphus of Sweden intervened in the Thirty Years' War. On 16 September 1631, Gustavus Adolphus' forces defeated Tilly's forces at the Battle of Breitenfeld (1631). Tilly was defeated the following year, and Gustavus Adolphus' forces swept into southern Germany. When Oppenheim was captured in December 1631, Frederick believed the time was ripe for him to reestablish himself in the Palatinate, and he left for Heidelberg.