French successes in 1690 had checked the Allies on most of the mainland fronts, yet their victories had not broken the Grand Alliance. With the hope of unhinging the coalition French commanders in 1691 prepared for an early double-blow: the capture of Mons in the Spanish Netherlands, and Nice in northern Italy. Boufflers invested Mons on 15 March with some 46,000 men, while Luxembourg commanded a similar force of observation. After some of the most intense fighting of all of Louis XIV's wars the town inevitably capitulated on 8 April. Luxembourg proceeded to take Halle at the end of May, while Boufflers bombarded Liège; but these acts proved to have no political nor strategic consequence. The final action of note in the Low Countries came on 19 September when Luxembourg's cavalry surprised and defeated the rear of the Allied forces in a minor action near Leuze. Now that the defence of the Spanish Netherlands depended almost wholly on the Allies William III insisted on replacing its Spanish governor, the Marquis of Gastañaga, with the Elector of Bavaria, thus overcoming delays in getting decisions from Madrid.