On 1 December 1941, "I-123" departed Samah along with "I-124" to begin what would become her first war patrol. Attached to the Philippines Seizure Force, the two submarines proceeded to the Philippines to support the impending Japanese invasion there. While at sea, "I-123" received the message "Climb Mount Niitaka 1208" () from the Combined Fleet on 2 December 1941, indicating that war with the Allies would commence on 8 December 1941 Japan time (7 December 1941 on the other side of the International Date Line in Hawaii, where the war would begin with Japan's attack on Pearl Harbor).