 He impressed the captured troops into his service and with them participated at the Swedish victory at battle of Chemnitz.By 1642, Douglas served in the army of Field Marshal Lennart Torstensson under whom he campaigned in Silesia before participating at the battle of Leipzig where he was accredited with playing a significant part in the victory.For this and other services Douglas was promoted major general, though not without protest from some of his German co-commanders.