In the wake of the 1939 German invasion of Poland, which sparked World War II, Belgium mobilized and Dom Bruno was assigned to be the chaplain of the 41st Artillery Regiment. In May of the following year, German troops overran Belgium. In the course of the campaign, he sustained a leg injury and spent the next six months in prisoner-of-war camps at Wolfsburg and Doessel, Germany, where he continued to provide religious and moral support to fellow prisoners. Upon his release, Dom Bruno returned to Mont-César in German occupied Belgium and resumed his teaching activities.