The War in Papua: The Strategic Context
Why was Port Moresby important?
Port Moresby was important because any Allied attack north through New Guinea towards Rabaul required Port Moresby as a base. Similarly for any attack south towards Australia, the Japanese required Port Moresby...
Amphibious operations - the sending of armies across the sea to invade another land mass- require a base from which to prepare and launch the operation. The closer the base is to the objective the better. If it is too far away then the operation will fall short in two key requirements: First, Fighter aircraft are required to fly air cover over the invading fleet before, during and after the amphibious landing. Fighters in 1942 had a short range so needed airfields close to the objective. Secondly, transport ships must, after the invasion has occurred, constantly shuttle back and forth from the base to the landing point to provide supplies, equipment and reinforcements. If the base is too far from the landing point the turnaround time will be too great and an impossible number of ships will be required to ensure the invasion force builds up its strength more rapidly than the defenders can build up theirs.
Video: Veteran Interview
... Kokoda veteran Gordon Bailey describes the reaction of the Australians in Port Moresby to the shooting down of a Japanese aircraft...
In 1942 the Japanese were capable of launching amphibious operations in the south-west Pacific up to a maximum of 800 kilometres away from their base at Rabaul. This is also the distance from Port Moresby to a suitable invasion site in north Queensland: Cairns.
A useful base required a harbour and docks, airfields, and critical infrastructure like storage sheds and a guaranteed water supply. Much of this did not exist when the Australians first arrived in Port Moresby early in 1941, but within a year the town was transformed into the only suitable site in Papua capable of supporting large amphibious operations. As such it was vital for the Australians to hold it if they were to advance northwards. Similarly, even if the Japanese did not intend to invade the eastern coast of Australia, holding Port Moresby allowed them to threaten to do so one day. Without Port Moresby the threat was a hollow one.