When the Second World War broke out, only limited resources where allotted to the defence of Northern Sweden, but following the Soviet attack on Finland a few months later and the German Operation Weserübung against Norway and Denmark the next year, major work on improving and extending the defensive lines was commenced. Bunkers, bomb shelters, gun emplacements—particularly for anti-tank guns—were built by the units that had been mobilised and stationed in the vicinity of Boden. The Finnish experiences gained from meeting superior armoured forces during the Winter War were adopted in Sweden, and triple rows of large stones or blocks of reinforced concrete, dragon's teeth, soon formed a continuous line—defended by anti-tank guns in concrete bunkers and machine-gun nests—around the fortress and city. As the war continued and advances in most fields of warfare were made, Boden Fortress was continually improved to meet new or increased threats such as air, armoured or gas attacks. The command, supply and intelligence services were improved as well.