In this capacity, he led the Secret Army, the largest faction of the resistance, from January 1944.

He was admitted to the Ecole de Guerre and served in the staff of the 3rd Brigade during the German invasion of Belgium in World War I.

He retired in April 1939 as a lieutenant general, before being recalled in September when the outbreak of World War II led to the mobilisation of the Belgian Army, despite its neutral status.

Following the arrest of Jules Bastin and his successor's escape to the United Kingdom, Pire became head of the Secret Army in January 1944.

In this role, he led the group's mobilisation after the Normandy Landings and during the Liberation of Belgium from June to September 1944.