In 1914, Belgium was invaded by the German Empire and placed under German military occupation. How involved Pirenne was in the Belgian resistance during World War I is not known. What is known is that Pirenne was questioned by German occupiers on 18 March 1916, and subsequently arrested. The occupying army had ordered striking professors at the University of Ghent to continue teaching. Pirenne's son Pierre had been killed in the fighting at the Battle of the Yser in October 1914. The German officer questioning Pirenne asked why he insisted on answering in French when it was known that Pirenne spoke excellent German and had done postgraduate studies at Leipzig and Berlin. Pirenne responded: "I have forgotten German since 3 August 1914," the date of the German invasion of Belgium, part of Germany's war plan to defeat France.