He was assigned leader of the rebellious Reisers by Philipp van Artevelde, and accountable for the victories against Count Louis II of Flanders.
While he travelled to England, in an attempt to find support for Flanders against the attacking French, Artevelde died in the Battle of Roosebeke and all Flanders but Gent was subdued by Count Louis. Back in Ghent, Ackerman took the place of Artevelde and fought victoriously in the Battle of Dunkerque. Even though the count died in 1384, the war was not over. Ackerman conquered the city of Damm and was soon beleaguered by the army of Charles VI of France. The expected help from the English failed to appear, and he had to break the siege on his own.