Codebreaking in New Guinea, 1944

From Hell's Battlefield: The Australians in New Guinea in World War II, by Phillip Bradley (Allen & Unwin, 2012), Kindle Loc. 6019-37:

On 20 March [1944], Emirau Island, 120 kilometres northwest of Kavieng, was occupied unopposed, and by the end of April two airfields had been constructed there. With Kavieng and Rabaul isolated, MacArthur could now make a great bound towards the Philippines. Having convinced the Joint Chiefs of Staff that Wewak should be bypassed, he planned to strike Hollandia (modern-day Jayapura), just across the border from Wewak in Netherlands New Guinea. Apart from isolating the Japanese Army in New Guinea, MacArthur wanted the prime anchorage of Humboldt Bay and the Lake Sentani airfields for his drive towards Japan.

Intelligence made the Hollandia decision possible. ULTRA decrypts, the decoded Japanese naval and Army communications, had already played an important part in New Guinea operations. ULTRA’s first success had been to expose Japanese intentions during the Papuan campaign, particularly the planned invasions of Port Moresby and Milne Bay. Later plans to reinforce Lae had been uncovered by ULTRA and then undone by the Battle of the Bismarck Sea. ULTRA had then kept MacArthur informed of the air buildup at Wewak, which had been so efficiently nullified by Kenney’s air arm. Now it gave MacArthur the priceless advantage of knowing that Hansa Bay was being reinforced and would be a tough nut to crack. The same was true of Wewak, but the decrypts confirmed that both Aitape and Hollandia were weakly held. The Japanese commanders were thinking in small steps, while MacArthur was planning a great leap.

The Australians played a major part in this intelligence coup. When the radio platoon from the Japanese 20th Division headquarters had pulled out from Sio in the wake of the Australian advance, its men had to carry the heavy components of the radios. However, a large trunk containing all their code books and other cipher material was left behind, buried in a nearby creek. It was discovered by Australian sappers sweeping the former headquarters site for mines and sent back to Australia, where the documents were painstakingly dried out and analysed. The cipher keys gave the Allies access to crucial intelligence on Japanese Army strength and plans in New Guinea.

So MacArthur would boldly strike for Hollandia six months ahead of the originally scheduled date. Though the operation’s code name, Reckless, may have indicated otherwise, MacArthur had the intelligence and the resources to succeed.