Siege of Leuven 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 . Background . In 1635 the Dutch Republic concluded an alliance with France with the objective of taking on the Spanish 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 village 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 . Siege . First operations . On 20 June the Franco - Dutch army lifted his camp and advanced its lines to the eastern bank of the Dijle river . Francisco de Moncada , 3rd Marquis of Aitona , commanding the Spanish Tercio of Marquis of Celada , several other tercios , and the cavalry under John VIII of Nassau - Siegen , proceeded to reinforce the German garrison of a fortified bridge over the Dijle fearing that the invading army use it to cross the river . The Spanish troops spent two hours observing the Franco - Dutch forces moving over the hills of the opposite riverside , when they discovered that they were crossing the Dijle using an undefended footbridge located a league from the first one . Duke of Lerma was immediately sent to avoid the crossing , in command of some cavalry under Juan de Vivero and 300 musketeers of Celada ' s tercio led by Captain Antonio de Velandia . By the time they arrived , more than 4 , 000 Franco - Dutch had passed the bridge and occupied strong defensive positions , as they had begun the maneuver at dawn of the same day . Seeing this , the Duke of Lerma ordered Celada to withdraw , warning him via Captain Diego de Luna that he couldn ' t relieve him if his men were attacked , as the cavalry under Johann von Nassau - Siegen had lagged behind . Siege . That night all the artillery and baggage marched back to Brussels , and at the following day , so did the Cardinal - Infante with his camp , leaving the defense of Leuven in charge of the veteran Anthonie Schetz , baron of Grobbendonck . He was in command of the Tercio of his son , Baron of Wezemaal , the Walloon Tercio of Ribacourt , and the Irish Tercio of Thomas Preston , besides five regiments of armed bourgeois and students of the University of Leuven , and some cavalry units . The Franco - Dutch army , having crossed the Dijle , looted the village of Tervuren , residence of the Dukes of Brabant , and reached Brussels ' ouskirts . They soon turned back to Leuven to invest the town . Siege warfare ensued , with the allied artillery firing over Leuven ' s fortifications and the pioneers driving trench es and mine s toward the rampart s . Most of the works were carried out at the gate of Vilvoorde , defended by Preston with his Irish tercio , whose numerous sorties to obstruct the siege works easily demoralized the poorly - supplied French soldiers . Walloon troops and students of the University made also many sorties , and the sap works were destroyed regularly every night . The besiegers , irritated by the obstruction to their works , decided to storm the ramparts even exposed to the enemy fire , taking advantage of their numerical superiority . Three regiments fell one night over the ramparts and bulwark s from the most advanced entrenchements , but were bloodily repelled by the wakeful defenders . The following night , Frederick Henry in person led an assault on the ravelin which protected the gate of Mechelen , guarded by just a handful of Irish . Despite the initial success of the attack , the Irish , aided by some Germans and bourgeois , managed to reject Frederick Henry inflicting him serious losses . Much of the failure was caused by the fact that the maneuvers of his army were being observed from a well - fortified tower , situated between the gates of Mechelen and Vilvoorde , popularly called " Verlooren - Kost " . This tower , besides as artillery position , served as look - out to Baron of Grobbendonck . When the Franco - Dutch realized this , the " Verlooren - Kost " was put under hevay artillery fire , but the 9 meters thick walls of the tower could not be overthrown . Relief . On 29 June , Feast of Saints Peter and Paul , while the Franco - Dutch remained inactive , Grobbendonck ordered 250 selected defenders to make a sortie . They went out from three different gates and met in front of the " Verlooren - Kost " tower . Then they stormed the besieging entrenchements by surprise , taking the troops occupying them completely unprepared . About 400 men , including a large number of officers , were killed . Despite the setback he suffered , Frederick Henry urged Grobbendonck that same day to surrender , threatening to massacre the inhabitants of the town . Five days later a relief army under Ottavio Piccolomini of 8 , 000 cavalry troops and the tercios of Alonso Ladrón and Sigismondo Sfondrati , which had been garrisoned at Namur to recover the Les Avins ' defeat , and a rearguard of 3 , 000 infantry and cavalry troops arrived in the outskirts of Leuven . Their presence forced the Franco - Dutch army , which were then suffering food shortages , to lift the siege and retreat north towards the United Provinces . A large number of soldiers deserted and were killed or captured by the Spanish cavalry and the Flemish peasants . Shortly afterwards the Cardinal - Infante also made appearance in command of 22 , 000 infantry and 14 , 000 cavalry men . Aftermath . The Franco - Dutch failure in front of Leuven ' s walls allowed the Spanish to take the initiative . The Cardinal - Infante counter - attacked , pushing the Franco - Dutch army back to the Dutch border . He made a north - easterly thrust to the Rhine in the direction of Cleves , recapturing Diest and Tienen . A party of 500 German mercenaries under Lt . - Col . Eyndhouts , roaming on his left flank , managed to surprise the Dutch fortress of Schenkenschans that at the time had a garrison of only 120 soldiers , in the night of 27 / 28 July . A large garrison were put then in the fortress , at first under the command of Eyndhouts . The Dutch brought up reinforcements right away , but could not prevent the occupation by a Spanish army of 20 , 000 of the Duchy of Cleves during August and September , with the aim of linking the fort of Shenck with the main body of the Spanish Netherlands . This army threatened an invasion of the Dutch heartland and it was therefore essential that this threat be countered . Frederick Henry personally started the siege of Schenkenschans within days of its fall , but soon transferred command to his cousin John Maurice , Prince of Nassau - Siegen . The fort fell after a long and costly siege that lasted even through the winter months . The Spanish turned then their attention against the French , invading the north of France towards the Somme and reaching as far as Corbie .