In 1627 he was received as a page in the Swedish service under John Casimir, Count Palatine of Kleeburg, castellan of Stegeborg Castle, brother-in-law of King Gustav II Adolf.

Then serving as a lieutenant colonel, Douglas had been tasked with the defence of Egeln in Saxony, and so was left very much within the territory of the newly hostile Elector Johan Georg who had defected to the Empire.

Douglas spectacularly broke out rather than change sides and managed to return with most of his men through hostile lines to Swedish controlled territory[5] He participated in the Battle of Wittstock the following year, now with the rank of full colonel and actually in command of two cavalry regiments.

With these he participated in the famous flanking charge led by Lieutenant General James King, the senior cavalry commander in Alexander Leslie's Army of the Weser.

It was during this period that the Scot captured the Imperial Fälttygmästare (master of ordinance) Salis and his regiments, who were heading towards Eger (Cheb) in Bohemia.

[6] 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.

At the battle of Jankowitz in 1645, Douglas commanded the left wing of the Swedish army, with some twenty-three out of forty-seven of the cavalry units on the field under his control.

However, when Douglas returned to Sweden in June to brief the Riksråd on the progress of negotiations, he delivered the news that the Bavarian truce had effectively expired.

[9] For his good offices as both soldier and negotiator, Douglas was promoted to the rank of lieutenant general at which station he saw out the remainder of the war, seeing his last main action at Zusmarshausen in May 1648.