In 1635 the Dutch Republic concluded an alliance with France with the objective of taking on the Army of Flanders from two sides, in the hope of breaking the strategic stalemate in the Eighty Years' War and dividing up the Spanish Netherlands between the two partners in the alliance. The French invaded from the south and defeated the Spanish army at Les Avins on 20 May, eventually joining forces in Maastricht with Frederick Henry of Orange, who had departed the Dutch Republic in command of 20,000 infantry and 6,000 cavalry soldiers,. Meanwhile, the Cardinal-Infante Ferdinand, who was in Leuven, ordered the Spanish Tercios to move to Tienen and sent the Count of Fuenclara to Germany with instructions to call the Imperial army for help.
The joint army of France and the United Provinces, then numbering 50,000 men, composed by French, Dutch, German and English soldiers, marched to Tienen, defended by a small garrison under Captain Martín de los Alarcos. The town was taken by assault, looted for three days, and finally razed. The Spanish garrison and most of its inhabitants were massacred. This event gave Ferdinand time to improve the fortifications of Leuven and to camp his army in a fortified position next to the city. The Franco-Dutch army made its appearance soon after and camped two leagues from Ferdinand's headquarters. Nevertheless, they remained inactive for eight days, which allowed the populace of all the country, but specially of Brussels, where the news of the sack of Tienen had caused great fear, to escape to safer places.