Siege of Mons ( 1691 ) The Siege of Mons , 15 March – 10 April 1691 , was a major operation fought during the Nine Years ' War , and was the main French objective for the 1691 campaign in the Spanish Netherlands . The city was besieged and captured before the normal commencement of the campaigning season with minimal losses . The outcome was not in doubt , but in a conflict dominated by siege warfare , neither the French army of King Louis XIV , nor the forces of the Grand Alliance under King William III , could bring about a decisive battle . After the siege the duc de Boufflers bombarded the neutral city of Liege , whilst the duc de Luxembourg captured Halle , and scored a minor victory against the Prince of Waldeck at the Battle of Leuze in September . Strategically , however , little had changed in the war , and both combatants returned to winter quarters at the end of the campaigning season . Background . French forces had secured considerable success in 1690 . In July Luxembourg fought and won his tactical masterpiece at the Battle of Fleurus , nullifying any Allied hopes of invading France , whilst at sea , Admiral Tourville defeated an Anglo - Dutch fleet off Beachy Head . In August Catinat had triumphed at the Battle of Staffarda in northern Italy ( although his force was too small to gain any strategic advantage ) . The only bright spot for the Grand Alliance in 1690 was King William ’ s victory over James II in Ireland at the Battle of the Boyne . Yet despite the battlefield successes , French forces in 1690 had failed to break the coalition ranged against the ambitions of King Louis . In 1691 the French had planned for a double strike : Nice in northern Italy , and Mons in the Spanish Netherlands . The Netherlands were again where France would concentrate its main war effort , and was a theatre where Louis ’ war minister , Louvois , had striven to bring together an even larger army than had been assembled the year before . These attacks on Nice and Mons were planned for very early in the campaign season , illustrating Vauban ’ s dictum that " It is a very favourable circumstance to be able to attack before the enemy takes the field in strength … " Meanwhile in Ireland the war continued into 1691 , but William now felt secure enough on his new throne in the British Isles to return to the war on the Continent . William entered The Hague on 5 February to organise his army for the coming campaign . After securing forces totalling 220 , 000 men , the Stadtholder - King retired to his country home . In mid - March , surrounded by representatives of the Grand Alliance , he received news that Mons was under siege . Siege . Louvois engineered the considerable preparations for the siege throughout the preceding winter : stores were filled with supplies in Namur , Philippeville , Dinant and Givet , and no less than 21 , 000 labourers were gathered for the construction of the lines of circumvallation . Louis , accompanied by members of his court , joined his army in the Spanish Netherlands to take control of the armies in theatre , arriving at the front on 21 March . The King ’ s besieging army of 46 , 000 ( under the direction of his great engineer , Vauban ) surrounded the town and its garrison of some 4 , 800 men . The Allies had formed an army of 38 , 000 under William to relieve the city , but Luxembourg ’ s army of observation , also 46 , 000 strong , denied the Allies any possibility of disrupting the operation . Marshal Boufflers began the investment on 15 March ; the trenches were opened ten days later . In one of the most intense attacks of all King Louis ’ wars , two batteries , each consisting of 12 mortars , bombarded the city in preparation for the assault ; by 30 March , the French had fired 7 , 000 cannonballs and 3 , 000 mortar shells . Inevitably , at 17 : 00 on 8 April , the besieged inhabitants beat the chamade ; conditions were settled , and the remaining men of the garrison marched out two days later . Aftermath . The siege had begun and ended before the normal commencement of campaigning . Louis returned to Versailles on 12 April , whilst William , after distributing his troops to various garrisons , returned to The Hague . The French now prepared for the rest of the 1691 campaign season with the creation of five large armies bound for five major fronts : Flanders , the Moselle , the Rhine , Piedmont , and Roussillon . The largest of these forces , 49 battalions and 140 squadrons under the command Luxembourg , took station in Flanders , but little was accomplished after the siege by either the French or the Grand Alliance . Luxembourg devastated Halle at the end of May , whilst Boufflers bombarded neutral Liege in early June , but these aggressive acts had no political results . Louis ’ personal military advisor and expert in the art of war , the Marquis de Chamlay , argued that these victories should be followed by a field battle that would destroy the Allied army and force a conclusion to the conflict . Louvois , however , suggested a bombardment of Brussels would force the issue , but was opposed by Luxembourg and Vauban . William , meanwhile , arrived at Anderlecht on 2 June to take command of the Allied army of 63 battalions and 180 squadrons , totalling 56 , 000 men . Luxembourg successfully manoeuvred to prevent William besieging Dinant , but subsequent manoeuvres produced little action . After William left his troops in the command of the Prince of Waldeck , Luxembourg ’ s cavalry routed part of the Allied army at Leuze on 19 September , before all combatants returned to winter quarters .