At the outbreak of World War II, Brandi continued to serve on minesweeper "M-1" under Bartels. "M-1" transported the "Marinestosstruppkompanie", a reinforced naval infantry platoon, to the battleship prior to the attack of the Polish base at Danzig's Westerplatte in the early morning hours of 1 September 1939. On 1 October 1939, he was promoted to "Oberleutnant zur See" (First Lieutenant). Following the Invasion of Poland, "M-1" undertook various minefield clearing operations in the North and Baltic Sea. On 24 February 1940, without prior warning, "M-1" rammed and sank four Esbjerg based Danish trawlers, "Ejjam" (E 92), "Gerlis" (E 456), "Mercator" (E 348) and "Polaris" (E 504) in the vicinity of the Dogger Bank. Bartels reported to his superiors that no one was rescued due to "military reasons"; 16 fishermen from the then neutral Denmark lost their lives. In April 1940 on "M-1", Brandi participated in Operation "Weserübung", Germany's assault on Denmark and Norway, and was awarded both classes of the Iron Cross ().