[9] 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.

[16] 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.

[16] 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.

[19] 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.

[21] 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.

[23] 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.

[22] 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.

[26] 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.

[26] 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.

[29] 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.

[29] The Spanish cavalry, alerted by the shots, made soon appearance led by Francisco Pardo and overran the French troops, who threw their weapons down asking for mercy.

[4] The loss on the Spanish side was light and consisted of 2 captains, Marco Antonio Felice and Count Evandro Piccolomini, nephew of the Imperial marshall, and 5 soldiers killed.

[4] Thomas of Carignan, having recognized the extensive French works around the Fort of Bacq, decided to encamp his army in the meadows of the area to avoid a pitched battle.

[31] Although Saint-Omer had been relieved, the pressure over the town increased, and at the same time the Dutch States Army captured one of the major Spanish forts defending Antwerp.

[22] During those days one of La Force officers, Sieur de Lermont, began to work a fort in a levee coming from Ardres to secure definitely Châtillon's corps supplies.

[22] It was promptly captured by the Spanish, and Châtillon, pressed by its loss, asked Louis XIII to approach La Force's corps to Saint-Omer to tighten the siege over the town.

[22] Thomas of Carignano sent Johann von Nassau-Siegen in command of most of the Spanish cavalry and a flying squadron of all the tercios under Maestre de Campo Francesco de Toralto to expel the French troops from a newly built redoubt which defended a levee in Hennuin near La Force's position, but they retreated considering that the levee was too close to attack without carrying boards as protection against the musketry fire.

[36] The night of 2 July, several soldiers under Captain don Rodrigo de Rojas, of Velada's tercio, made a sortie against Du Hallier works, but were rejected by the Scottish Regiment of Colonel Lord James Douglas.

[37] He feared a revolt of the population of the town against the garrison, and although the Bishop and the Abbot of St. Bertin de Clairmarais placated them, it was suspected that somebody within the city maintained contact with the French.

[38] The prince informed Saavedra that he would find 2,000 fagots to cover the front of the trenches at the bridge of Sainte-Marie-Kerque and told him that most of the cavalry under Johann von Nassau-Siegen and 2,000 infantry commanded by Count of Fuensaldaña and Francesco de Toralto would isolate St. Jean from any force of relief.

[33] The Spanish cavalry was in danger of being disbanded for a moment, but a sleeve of musketeers under Captain Don Antonio Pimentel, hidden in some hedges, managed to hold back the attack giving time to the Count to withdraw his troops.

Fearing that the relief force arrived on time, the Maestro de Campo sent his aid to ask Thomas of Carignano for permission to assault the fort, which was given.

Most of the combined Imperial-Spanish cavalry was dispatched under the Count of Nassau-Siegen, Francisco Pardo, and the Imperial General Girolamo Colloredo to prevent La Force from joining his troops with Châtillon.

The explorers of the contoy reported news of the advance of the Imperial-Spanish cavalry under Count von Nassau-Siegen and General Colloredo, whose strength was put in 4,000 men, through the levee of Hennin, near Ruminghem.

Moreover, a French patrol guarding the area between Bacq and Du Hallier's quarter captured two disguised men attempting to reach the Spanish army, probably to inform them that the preparations into Saint-Omer were ready.

A second fort, known as Esquenque, surrendered to Francesco de Toralto, allowing its garrison of 600 men to return to their army because the lack of ammunitions prevented them to organize a determined resistance.

As soon as the Regiments of Piémont and de la Marine under Compte d'Arpajoux had passed the hedges, the Imperial-Spanish cavalry, forming in 12 squadrons, attacked them with great fierceness.

Piccolomini, meanwhile, had taken a redoubt by assault and was attacking the Church of Saint-Momelin, and Owen Roe O'Neill had captured the French outpost in the riverside near Watten, which allowed him to introduce supplies into Saint-Omer with the help of an officer sent from the town.

The Prince of Carignano had finally relieved Saint-Omer, where he met the Baron of Wezemaal, Lancelot of Grobbendonk, and was informed by Piccolomini's envoy, the Marquis of Gonzaga, that the garrison of Saint Momelin had offered its surrender, which he accepted.

Simultaneously, the Spanish soldiers were ordered to return each one to his tercio, and the following day the army marched to Térouanne led by the Prince himself, who reviewed the troops before he went to Brussels to inform the Cardinal-Infante of his success.