Japanese aircraft soon discovered the Allied presence at Milne Bay, which was appreciated as a clear threat to Japanese plans for another seaborne advance on Port Moresby, which was to start with a landing at Samarai Island in the China Strait, not far from Milne Bay.
On 31 July the commander of the Japanese XVII Army, Lieutenant General Harukichi Hyakutake, requested that Vice Admiral Gunichi Mikawa's 8th Fleet capture the new Allied base at Milne Bay instead.
Mikawa therefore altered his plans for the Samarai operation, and substituted the capture of Milne Bay, which was codenamed Operation RE, and scheduled for the middle of August.
Operation RE received a high priority after aircraft from the 25th Air Flotilla discovered the new Milne Bay airfields on 4 August, but was then postponed due to the American landings on Guadalcanal on 7 August.

Under the misconception that the airfields were defended by only two or three companies of Australian infantry (300–600 men), the initial Japanese assault force consisted of only about 1,250 personnel.
The Imperial Japanese Army (IJA) was unwilling to conduct the operation as it feared that landing barges sent to the area would be attacked by Allied aircraft.
Following an argument between IJA and Imperial Japanese Navy (IJN) officers, it was agreed that the Navy would have responsibility for the landing.
As a result, the assault force was drawn from the Japanese naval infantry, known as Kaigun Rikusentai (Special Naval Landing Forces).
Some 612 naval troops from the 5th Kure Special Naval Landing Force (SNLF), led by Commander Masajiro Hayashi, were scheduled to land on the east coast near a point identified by the Japanese as "Rabi", along with 197 men from the 5th Sasebo SNLF, led by Lieutenant Fujikawa.
It was planned that a further 350 personnel from the 10th Naval Landing Force, along with 100 men from the 2nd Air Advance Party, would land via barge on the northern coast of the peninsula at Taupota, in Goodenough Bay, from where it would strike out over the Stirling Ranges to attack the Australians from behind.
Following the battle, the chief of staff of the Japanese Combined Fleet, Vice Admiral Matome Ugaki, assessed that the landing force was not of a high calibre as it contained many 30- to 35-year-old soldiers who were not fully fit and had "inferior fighting spirit".
Naval support was to be provided by the 18th Cruiser Division under the command of Rear Admiral Mitsuharu Matsuyama.
The Japanese enjoyed some initial advantage in the form of possessing two Type-95 light tanks.
After an initial attack, however, these tanks became marooned in the mud and abandoned.
They also had control of the sea during the night, allowing reinforcement and evacuation.
Japanese aircraft soon discovered the Allied presence at Milne Bay, which was appreciated as a clear threat to Japanese plans for another seaborne advance on Port Moresby, which was to start with a landing at Samarai Island in the China Strait, not far from Milne Bay. On 31 July the commander of the Japanese XVII Army, Lieutenant General Harukichi Hyakutake, requested that Vice Admiral Gunichi Mikawa's 8th Fleet capture the new Allied base at Milne Bay instead. Mikawa therefore altered his plans for the Samarai operation, and substituted the capture of Milne Bay, which was codenamed Operation RE, and scheduled for the middle of August. Operation RE received a high priority after aircraft from the 25th Air Flotilla discovered the new Milne Bay airfields on 4 August, but was then postponed due to the American landings on Guadalcanal on 7 August.
Under the misconception that the airfields were defended by only two or three companies of Australian infantry (300–600 men), the initial Japanese assault force consisted of only about 1,250 personnel. The Imperial Japanese Army (IJA) was unwilling to conduct the operation as it feared that landing barges sent to the area would be attacked by Allied aircraft. Following an argument between IJA and Imperial Japanese Navy (IJN) officers, it was agreed that the Navy would have responsibility for the landing. As a result, the assault force was drawn from the Japanese naval infantry, known as Kaigun Rikusentai (Special Naval Landing Forces). Some 612 naval troops from the 5th Kure Special Naval Landing Force (SNLF), led by Commander Masajiro Hayashi, were scheduled to land on the east coast near a point identified by the Japanese as "Rabi", along with 197 men from the 5th Sasebo SNLF, led by Lieutenant Fujikawa. It was planned that a further 350 personnel from the 10th Naval Landing Force, along with 100 men from the 2nd Air Advance Party, would land via barge on the northern coast of the peninsula at Taupota, in Goodenough Bay, from where it would strike out over the Stirling Ranges to attack the Australians from behind. Following the battle, the chief of staff of the Japanese Combined Fleet, Vice Admiral Matome Ugaki, assessed that the landing force was not of a high calibre as it contained many 30- to 35-year-old soldiers who were not fully fit and had "inferior fighting spirit". Naval support was to be provided by the 18th Cruiser Division under the command of Rear Admiral Mitsuharu Matsuyama. The Japanese enjoyed some initial advantage in the form of possessing two Type-95 light tanks. After an initial attack, however, these tanks became marooned in the mud and abandoned. They also had control of the sea during the night, allowing reinforcement and evacuation.
Battle of Milne Bay