The Siege of Leuven (June 24 – July 4, 1635) was an important siege in the Thirty Years' War in which a Franco-Dutch army under Frederick Henry of Orange and the French Marshals Urbain de Maillé-Brezé and Gaspard III de Coligny, who had invaded the Spanish Netherlands from two sides, laid siege to the Spanish city of Leuven, defended by 4,000 Flemish civilians, Walloons, Spaniards and Irish under Anthonie Schetz, Baron of Grobbendonck. Poor organization and logistics and the spread of sickness among the French, along with the appearance of a relief army under Ottavio Piccolomini, forced the invading army to lift the siege. This failure allowed the Spanish forces to take the initiative and soon the invaders were forced into a headlong retreat.