Spanish Road
The reason outside powers became involved in what was an internal German dispute was the imminent expiry of the 1609 Twelve Years' Truce, which suspended the war between Spain and the Dutch. Before restarting hostilities, Ambrosio Spinola, commander in the Spanish Netherlands, needed to secure the Spanish Road, an overland route connecting Habsburg possessions in Italy to Flanders. This allowed him to move troops and supplies by road, rather than sea where the Dutch navy was dominant; by 1618, the only part not controlled by Spain ran through the Electoral Palatinate.