In 1635 France and the Dutch Republic formed an alliance against Spain with the objective of conquering and partitioning the Spanish Netherlands. They invaded on two fronts in June 1635, but soon the Spanish forces regained the initiative against the combined Franco-Dutch army, which was ignominiously driven to the Dutch border. There Spain managed to capture the strategic fortress of Schenkenschans by surprise. This forced the Dutch to enter upon a long and costly siege of that fortress that occupied the Dutch army for nine months.