During March 1944, the Allies of World War II rapidly reinforced the military units located in the state of Western Australia to defend against the possibility that Japanese warships would attack the cities of Fremantle and Perth. This redeployment began on 8 March after concerns were raised about the purpose of Japanese warship movements near the Netherlands East Indies, and ended on 20 March, after it was concluded that an attack was unlikely.
In February 1944, the Allies became alarmed that the movement of the main Japanese fleet to Singapore could be a precursor to raids in the Indian Ocean, including against Western Australia. The emergency began when Allied code breakers detected the movement of a powerful force of Japanese warships in the Netherlands East Indies in early March. After a United States Navy submarine made radar contact with two Japanese warships near one of the entrances to the Indian Ocean on 6 March, the Allied military authorities and Australian Government judged that a fleet may have been heading towards the Perth area. In reality, these warships were undertaking a patrol while awaiting a small raiding force to return from attacking ships in the central Indian Ocean.
In response to the perceived threat, the Allied military units stationed in Western Australia were placed on alert, and reinforcements were dispatched. These included six Royal Australian Air Force flying squadrons. Other Allied air units were held at Darwin in the Northern Territory to respond to raids on that town or reinforce Western Australia if the Japanese fleet was sighted. An air raid warning was sounded in Fremantle and Perth on 10 March, but this proved to be a false alarm. Intensive patrols by the Allied militaries did not detect any Japanese warships off Western Australia, and most units were stood down on 12 March. On the 20th of the month it was concluded that the threat of attack had passed, and the air reinforcements that had been sent to Western Australia returned to their bases.
Background
Allied defences
Fremantle, Western Australia, was an important port throughout World War II. From the start of the war, it was the assembly point for convoys of Allied ships travelling between Australia, the Middle East and Europe. Soon after the outbreak of the Pacific War, the city became a key operating and maintenance location for United States Navy submarines. The Fremantle submarine base was established in March 1942, and eventually became the US Navy's second-largest submarine base after Pearl Harbor in Hawaii. Royal Netherlands Navy submarines were also stationed at Fremantle from 1942, and the Royal Navy began transferring submarines there in mid-1944. Allied submarines operating from Fremantle played a key role in the offensive against Japanese shipping; over the course of the war, 154 submarines made 341 combat patrols from the port.
The Australian Government and local civilians regarded the geographically isolated Perth–Fremantle area as being vulnerable to attack. These fears peaked in March 1942 following the rapid Japanese advance through South East Asia during December 1941 and the early months of 1942. No Japanese attack on the region eventuated, though several small towns in the north of Western Australia were bombed. Concerns over Fremantle's security continued; in March 1943 Australian Prime Minister John Curtin noted as part of a message to British Prime Minister Winston Churchill and US President Franklin D. Roosevelt that the port was vulnerable to bombardment by Japanese warships or aircraft flying from aircraft carriers.