A strong admirer of Benito Mussolini, but equally demonstrating a staunch hatred of Germany, Hoornaert was a member of the far right veterans' group National Legion (, ) which was established in Liège in 1922. Initially a minor group it grew rapidly from 1927 when Hoornaert became leader and converted it into a paramilitary militia. Under Hoornaert's leadership the 4,000-strong group, who wore a blue-shirted uniform in common with other European fascist groups, became associated with a militant Belgian nationalism, rejecting emphasis on both Flemish and Walloon identities in favour of a united Belgium. Other important ideological aspects included anti-communism, opposition to liberalism, and the parliamentary system. As Legion leader Hoornaert participated in the 1934 Montreux Fascist conference, campaigned in support of the Italian invasion of Ethiopia (1935–37) and raised volunteers for the Nationalists in the Spanish Civil War (1936–39).