Aluminum plays a major role during the Second World War, as it is essential for the manufacture of airplanes. In the midst of hostilities and fearing aerial bombardment, England turns to Canada to supply aluminum. The Arvida smelter, just over ten years old, becomes a centre of war production.
To meet the strong demand, the Aluminum Company of Canada (Alcan) needs to expand and build. The great need for electricity required in the production of aluminum triggers the resumption of the Shipshaw hydroelectric project, which was interrupted during the economic crisis. An average of 3,900 men work there every day in two ten-hour shifts. Considered one of the world’s largest hydroelectric developments, the Shipshaw project is completed in only 18 months, an amazing feat. Simultaneously 19 new potrooms are built in Arvida in addition to the various facilities needed for alumina, carbon, fluorspar, transport, etc. Arvida Works becomes the world's largest aluminum complex.
During the war, between 80% and 90% of the Commonwealth's aluminum is produced in Arvida. Fearing possible sabotage, the company takes measures to ensure the protection of its facilities. A force of about 30 guards is formed, following which the government sends a company of the Home Guard, the Veterans Guard of Canada (VGC). A strategic location for the production of aluminum, the Saguenay is designated a vulnerable site requiring greater protection. In June 1941, the 14th Anti-Aircraft Artillery Battery arrives in the area with four guns, which at the time represent half of all heavy anti-aircraft artillery in Canada. Two are installed near the Isle-Maligne hydroelectric station and two others near the Arvida plant.
Gradually, the region is given greater protection with the addition of soldiers, machine guns and light rapid-fire artillery. At the same time, the Bagotville military airport is built to provide regional air defence. The peak of protection is reached in mid-1943, when the numerous soldiers of the 24th Anti-aircraft Regiment deploy 10 heavy guns and 22 light guns at strategic locations to protect the facilities of the Aluminum Company of Canada (Alcan).
The end of July 1944 marks the withdrawal of the guns and of most of the soldiers who had provided protection for Arvida. The remaining soldiers are withdrawn in December. A month later, Bagotville airport is closed. The war will have changed Arvida’s landscape with the expansion of the aluminum smelter, the construction of several hundred houses and various other buildings, including the Saguenay Inn and Brittany Row.