The siege of Saint-Omer (24 May – 16 July 1638) was a siege in the Thirty Years' War in which a French army under Gaspard III de Coligny, Maréchal de Châtillon, laid siege to the Flemish city of Saint-Omer, defended by a small garrison in command of Lancelot II Schetz, count of Grobbendonck. Despite several initial successes in the capture of the minor forts around Saint-Omer, on the night of 8/9 June a Spanish relief army under Thomas Francis, Prince of Carignano, surprised Châtillon's troops and established a small fort in the middle of the French lines. An entire army corps under Maréchal de La Force was ordered to move towards Saint-Omer to support Châtillon siege, but on 12 July a further Imperial-Spanish force commanded by Ottavio Piccolomini entered Saint-Omer, resolving the French marshals to withdraw.
Background
In June 1635 the allied armies of the Dutch Republic and France invaded the Spanish Netherlands from two sides and joined forces in the valley of the Meuse while the Spanish field army under the Cardinal-Infante fell back to cover Brussels. The invading armies captured a few smaller places before investing Leuven. The siege was a costly failure due to bad logistics and organization, and because the French army was decimated by the plague. The Cardinal-Infante was then able to counter-attack and pushed the Franco-Dutch army back to the Dutch border in the direction of Cleves, recapturing Diest and Tienen, and surprising the Dutch fortress of Schenkenschans, where a large garrison was put then in. The Duchy of Cleves was occupied during August and September with the aim of linking the fort with the main body of the Spanish Netherlands. while Frederick Henry, Prince of Orange, started the Siege of Schenkenschans. The fortress fell after a long and costly siege that lasted even through the winter months. Meanwhile, Cardinal Richelieu took the decision to remove Châtillon from command and focused France's effort in the Rhineland.
In the first months of 1636, while the French armies suffered further defeats against the Duke Charles of Lorraine and the Imperial generalissimo Matthias Gallas, the Count-Duke of Olivares insisted the Cardinal-Infante to continue concentrating the war effort in exploiting the gains in the Lower Rhine and in northern Brabant rather than in an offensive against France, even after the loss of Schenkenschans. In late May, however, the offensive operations were suspended and a secondary thrust was launched into France, according to the Cardinal-Infante, because of a dramatic change in circumstances. Philip IV wrote to his brother in June ordering him to advance into northern Brabant to try to recapture Schenkenschanz. Meanwhile, the Emperor, whose position in Germany had strengthened since the Peace of Prague, had projected an invasion of eastern France, but as his force was not large enough due to logistical problems, he proposed a joint invasion to the Cardinal-Infante.
On 4 July the Cardinal-Infante, commanding a lightly equipped army, crossed the frontier via Avesnes and took the fortresses of Le Catelet and La Capelle. Alarmed by this easy advance, the King of France Louis XIII was forced to move to Paris from Fontainebleau. The Spanish army, then led by Thomas Francis of Carignano, the commander of the Army of Flanders, successfully crossed the Somme and invested the vital fortress of Corbie, which surrendered a week later. Cardinal Richelieu had no other choice but to suspend the offensive against the Franche-Comté, ordering the army under Henri de Bourbon, Prince de Condé and Charles de La Porte de La Meilleraye to withdraw to defend Paris. Another one of Ferdinand's generals, Ottavio Piccolomini, tried to persuade the Cardinal-Infante to advance further into France, but Ferdinand considered that such operation could risk his army and soon retreated to Cambrai, before the Imperialist invasion had begun. The French armies regained most of the lost ground over the following months, including Corbie, an operation that absorbed the entire French army and was led directly by Richelieu.
Olivares, though distressed for the loss of Corbie, planned a renewed offensive against France for the following year, so Ferdinand began to mass his forces on the French border. In July Frederick Henry of Orange, seizing the moment, invested Breda in command of 22,000 soldiers. Garrisoned by 3,000 Spaniards, Italians, Wallons and Burgundians, Breda was one of the main fortresses of the Spanish Netherlands and a symbol of the Spanish power in Europe. A Spanish force under the Cardinal-Infante Ferdinand attempted to relieve the garrison of the city, but failed to dislodge the besiegers. Ferdinand decided move with his army to the valley of the Meuse, where he took Venlo and Roermond in order to distract Frederick Henry. He was, nevertheless, forced to turn back shortly after due to the French advances in Artois, Hainaut and Luxembourg, and Breda was captured by the Dutch on 7 October.
For the campaign of 1638, Philip IV instructed the Cardinal-Infante to undertake an offensive strategy against the Dutch in order to subject them to massive pressure and force them to agree a favourable truce and the restoration of their conquests in Brazil, Breda, Maastricht, Rheinberg and Orsoy. His main objective would be the capture of Rheinberg, which would give to Spain a crossing point in the Lower Rhine and contribute to tightening the blockade over Masstritch. Ferdinand was also ordered, when the offensive operations had finished, to quarter his army near the Dutch frontier to protect Antwerp, which had become vulnerable since the loss of Breda, and even to reinforce the garrisons of many secondary fortresses.
Siege
First operations
Châtillon, seeing that Saint-Omer was garrisoned just by 4 companies of the Tercio of José de Saavedra numbering 1,000 men and about 300 cavalry, while the town required more than 3,000 soldiers to be properly defended, resolved to besiege it. By 26 May his army had invested the town. Three days later the outpost of Arques was captured. Its defenders, 50 soldiers of the Walloon Tercio of Baron of Wezemaal and his officer, surrendered after a brief bombardment by the French artillery.
Having installed his headquarters in Arques, Châtillon proceeded to attack St. Bertin de Clairmarais Abbey, a good defensive position. Captains Lannoy and Dutally, of Wezemaal's tercio, hold back the French assaults until they lacked gunpowder, which forced them to surrender, even though in good conditions. The French Captain Campi of the Régiment de Navarre lost his arm during the fight. Châtillon's main objective was then to isolate Saint-Omer by blocking all the accesses to the town. With this aim he sent Sieur de La Ferté-Imbault to capture all the forts located between Ardres and Saint-Omer, which would also secure their own supplies, brought from Calais. The strongest fortresses, Dumenghen and Ennuin, could not be taken. Duc de la Force's troops had to be therefore employed to protect the supply convoys against possible ambushes.
Paul-Bernard de Fontaines, a Lorrainer in Spanish service who was governor of Bruges since 1629 and one of the oldest officers of the Army of Flanders, moved with his troops from Flanders to the village of Watten as soon as was informed of the French movements. From Watten Fontaine relieved Saint-Omer by sending to the town 4 companies of the Spanish Tercio of Marquis of Velada and 2 companies of the English Tercio of William Tresham, all of them commanded by Captain Luis de Mieses, who was ordered to take command of Saint-Omer's garrison for being the oldest officer in the town. Lancelot II, Count of Grobbendock, the former commander who was either in Saint-Omer, was given the order of defend the outpost of Bacq, a crucial position to receive relief from the Spanish lines since it controlled a channel of the river Aa giving access to the town. Châtillon was warned by many of his officers, Sieurs de La Barre, de Manican and Le Rasle, of the urgent need to take Bacq. The French marshall avoided a direct assault, but the fort was eventually occupied when Baron of Wezemaal, seeing the weakness of his position, decided to withdraw into Saint-Omer with his troops.
Having left a garrison under Sieur de Genlin in Bacq, Châtillon proceeded to capture 3 small forts defending the Canal de Neufossé to continue the circumvallation of Saint-Omer. The strongest of them, in command of Viscount Furnes, Great bailiff of Cassel, capitulated without opposing resistance. The other two, defended by the villeins of the Castellany, were taken by Châtillon, being that most of their defenders were slaughtered. French cavalry troops under Colonel Silar were then able to cross the Neufossé and enter Flanders, where they plundered Cassel and several other villages before returning with Châtillon. A great booty was obtained. Lieutenant General François de L'Hospital, Seigneur Du Hallier, meanwhile, had occupied the Abbey of Ouate. Fontaine, whose position in Watten had become insecure due to the loss of the forts around Saint-Omer, decided to retreat to Bergues-St. Vinocx after set fire to the village to avoid its use by the French. He later marched to Brussels to be appointed General of the artillery.
First relief
Count Ernest von Isenburg-Grenzau, having been ordered to move to Flanders diligently, departed Arleux with the Tercios of Carlo Guasco and Francesco Toralto, an artillery Lieutenant General with several artillery pieces, Don Juan de Vivero with some cavalry, and three companies of Wezemaal's tercio. He passed Béthune and arrived at Chocques, joining forces during the march with José de Saavedra's Tercio and its cavalry. They crossed the river Lys at Merville, and a day later reached Poperinge, where the Prince Thomas of Carignano took the command of the Spanish army. Most of his troops were quartered in Bourbourg and other nearby villages while he and his staff discussed how relieve Saint-Omer, a difficult task, as the majority of the forts around the town were occupied by the French, whose army was twice as large as the Spanish. Châtillon's soldiers, however, had not begun to dig trenches around the town yet because his commander was awaiting orders to do it when the Dutch States Army had launched its offensive against Antwerp. Despite his passive attitude, Saint-Omer's reduced garrison was in need of supplies and gunpowder.
Thomas of Carignano, seeing that the Imperial army under Ottavio Piccolomini could not arrive on time, resolved to relieve the town alone to enable its garrison to keep the resistance for longer. With this aim he disposed four flying squadrons, each one on 1,000 men, commanded respectively by Maestre de Campo Alfonso Pérez de Vivero, Count of Fuensaldaña, Maestre de Campo Owen Roe O'Neill, Sargento Mayor Dionisio de Guzmán of Fuensaldaña's tercio; and Maestro de Campo Francesco de Toralto. They were accompanied by the Neapolitan Tercio of Toralto, the tercio of Marquis de Velada under Sargento Mayor Juan Porcel, the German Regiments of Colonels Spinola and Rouveroy, the English Tercios of William Tresham and Henry Gage, Owen Roe O'Neill's Irish tercio under his Sargento mayor, José de Saavedra's Tercio divided in two squadrons, one of them under Saavadera himself and the other under'Sargento Mayor Diego López de Zúñiga, and 2,000 cavalrymen under Johann von Nassau-Siegen, who was accompanied by Juan de Vivero, Lieutenant General of Cavalry of the Army of the French Frontier and Fuensaldaña's brother, and a Croatian regiment under Colonel Ludovico. Count of Forjaz was sent across the Neufossé to attack the French troops at their quarters while Francisco de Pardo, Commissar General of the Cavalry of the Army of the French Frontier, remained in the rear with several battalions.
At the same time that the relief force marched to Saint-Omer, Thomas of Carignano sent Paolo Fanfanelli, Sargento Mayor of Carlo Guasco's tercio, and his soldiers, to occupy the tower of the Abbey of Watten, located on top of a hill and guarded by 50 French soldiers under an officer since Count Paul Bernard de Fontaine had abandoned the village. Lacking of supplies and discouraged by the size of the Spanish army, they promptly surrendered. Châtillon, thinking that Watten was a crucial position to control the riverside of the Neufossé, had dispatched the Regiments d'Espagni and de Fouquerolles over the village, but they arrived after the tower had surrendered.