When the Elector of Bavaria besieged Brussels, the regiment formed part of the force which marched to the relief of that city, passed the Scheldt, and carried the enemy's positions beyond that river on the 27th of November; which was followed by the retreat of the enemy from before Brussels.
The citadel of Lisle surrendered on the 9th of December; Ghent and Bruges were afterwards recaptured, and the regiment had its winter quarters at Ghent.
1709
Having reposed a few months in quarters, and received a body of recruits from England, the regiment traversed the conquered territory to Lisle, in June, 1709, and afterwards took part in the manœuvres by which Marshal Villars was induced to reduce the strength of his garrisons in his fortified towns, to reinforce a line of entrenchments and forts, in which he expected to be attacked. This object gained, the siege of Tournay was immediately commenced; and the Fifteenth foot, commanded by Lieut.-Colonel Andrew Armstrong, formed part of the covering army; but when the town surrendered, the regiment joined the besieging force, and took part in the attacks on the castle. This proved a desperate service. The citadel of Tournay was celebrated for the multiplicity of its under-ground works, and the approaches were carried on by sinking pits, and excavating subterraneous passages to the enemy's casemates and mines. The soldiers employed on these works were sometimes drowned with water, suffocated by smoke, and buried by explosions; and at other times parties of the besieging force and of the garrison met, and fought with sword and pistol in these gloomy labyrinths. In these services the Fifteenth regiment had a number of men killed and wounded; it also lost several men from the explosion of a mine, which destroyed a battery.
On the 3rd of September, the citadel of Tournay surrendered; and the army traversed the country towards Mons, the capital of the province of Hainault, leaving the Fifteenth and several other corps at Tournay, to level the approaches and fill the excavations. Immediately after this work was performed, the regiment traversed the country towards Mons, and joined the army, on the morning of the 11th of September, at the moment the columns of attack were advancing to assault the enemy's fortified position at Malplaquet. This proved one of the most sanguinary and hard-contested battles of the war: the confident and fierce attacks of the allies were made against formidable works, defended with resolution, which occasioned a great sacrifice of life; but eventually the position was forced, and the French army retreated with the loss of many colours, standards, cannon, and officers and soldiers made prisoners. The Fifteenth were in reserve on this occasion, and its loss was limited to Brevet Major Leslie, killed, and three or four private soldiers killed and wounded.
This victory was followed by the siege of Mons, and the regiment formed part of the covering army. The garrison surrendered in October.
On the 23rd of October, Major-General Howe was succeeded in the colonelcy of the regiment by Algernon Earl of Hertford, afterwards Duke of Somerset, who had served with reputation at several battles and sieges on the continent.
1710
The regiment quitted its winter quarters at Ghent, on the 14th of April, 1710, and marched to the rendezvous of the army near Tournay. The services of the Fifteenth foot were this year connected with the forcing of the French lines at Pont-à-Vendin, and with the siege and capture of Douay, which fortress surrendered on the 27th of June. They subsequently formed part of the covering army encamped at Villars-Brulin, during the siege of Bethune. This place having surrendered on the 29th of August, and the French army avoiding a general engagement, the fortresses of Aire and St. Venant were invested, and taken; and these conquests were the last important events of the campaign.