Rabaul New Britain; Wikimedia Commons
After Germany’s defeat in WWI, Australia occupied New Guinea with Rabaul as its administrative capitol. In January 1942, intending to separate Australia from U.S. forces by occupying New Guinea and the Solomon Islands, Japan seized Rabaul and transformed into a mighty military fortress. Miles of tunnels were dug for shelter from air attacks and barracks and support facilities were constructed to accommodate up to 110,000 Japanese troops by 1943. Almost impregnable, the Allies chose to cut the base off from its supplies (Rings around Rabaul) rather than directly attack it. Rabaul remained in Japanese control until the end of the war.