On the 1 September 1939 German forces invaded Poland. III./KG 3 attacked Polish positions defending the Tczew bridge. Despite the attack, and another from StG 1, the Poles repaired the wires and blew up the bridge two hours later. Both "Gruppen" were involved in the Battle of Grudziądz. They were involved in operations over the Narew, Praha. KG 3, paired with KG 27 under Wimmer, pushed southward from 6 September to isolate Warsaw from the east. They supported the advance on Warsaw and the subsequent siege. Both groups bombed targets in the Battle of Modlin. KG 3 losses are unknown, but II./KG 3 lost a bomber in mid-air explosion owing to a faulty fuze; with III./KG 4, and I./KG 152 from 3–6 September 1939. KG 3 bombed targets around Płock, Lida and Biała Podlaska. Both groups participated in the Battle of the Bzura and the destruction Army Poznań and Army Pomorze.