Between operations, Hahn returned to Berlin, where he was able to slip back to his old laboratory and assist Meitner with her research. In September 1917 he was one of three officers, disguised in Austrian uniforms, sent to the Isonzo front in Italy to find a suitable location for an attack, utilising newly developed rifled "minenwerfers" that simultaneously hurled hundreds of containers of poison gas onto enemy targets. They selected a site where the Italian trenches were sheltered in a deep valley so that a gas cloud would persist. The Battle of Caporetto broke through the Italian line and the Central Powers overran much of northern Italy. In 1918 the German offensive in the west smashed through the Allies' lines after a massive release of gas from their mortars. That summer Hahn was accidentally poisoned by phosgene while testing a new model gas mask. At the end of the war he was in the field in mufti on a secret mission to test a pot that heated and released a cloud of arsenicals.