From 1 March to 30 June 1939, Merten took an artillery officer's training course for battleship and was posted to Schleswig-Holstein in July 1939 as a cadet training officer. World War II began on 1 September 1939 when German forces invaded Poland. Merten participated in the bombardment of the Polish base at Danzig's Westerplatte in the early morning hours of 1 September 1939. On 7 September, he led Schleswig-Holstein's naval infantry troops in the Battle of Hel. Together with the 5. "Marineartillerie-Abteilung" (5th Naval Artillery Department), German troops landed on the Hel Peninsula. For his actions in these battles he received the Iron Cross 2nd Class on 2 October 1939.