Following the outbreak of the Pacific War in December 1941, the Japanese armed forces conducted military operations against U.S., British Commonwealth, and Dutch possessions in the Pacific and Southeast Asia. The first phase of these operations, which was the seizure of Malaysia, Singapore, the Dutch East Indies, the Philippines, and various island groups in the central and western Pacific, was virtually complete by March 1942. The second phase, initiated by Japanese Imperial Headquarters on January 23, was designed to isolate and neutralize Australia and India. In the Pacific, this plan envisioned the seizure of bases in Papua/New Guinea and the Solomon Islands, which would be used to support future operations against New Caledonia, Fiji, and Samoa. By early March, with the seizure of Lae and Salamaua, the entire north coast of Papua/New Guinea had fallen to Japanese forces who were planning for an amphibious invasion of Port Moresby.
===Early intellegence===
By this time, two secure American naval intelligence centers were in operation in the Pacific: one in Melbourne, Australia, and another at Pearl Harbor ("Hypo"). A third, at Corregidor ("Cast"), was rapidly disintegrating under Japanese air and artillery attacks on the island. Its cryptanalysts and equipment were in the process of evacuation to Melbourne. These facilities intercepted Japanese radio communications and, through traffic analysis and codebreaking, uncovered the location of major fleet units and shore based air forces. More importantly, by translating messages and studying operational patterns, Melbourne and Hypo predicted future Japanese operations. The intelligence centers provided their analysis, through daily communications intelligence (COMINT) briefings and warning reports, to senior American commanders, including Commander in Chief, U.S. Fleet, Admiral [[Ernest J. King]], and Commander in Chief, Pacific Fleet, Admiral [[Admiral Chester W. Nimitz]].
===Coral Sea===
In early March, the Japanese postponed their planned seizure of Port Moresby because of previous events. An American carrier raid on Japanese shipping at Lae and Salamaua on the 10th, along with a previously unsuccessful attempt to attack Rabaul on February 20, had demonstrated to Commander in Chief, Fourth Fleet (Admiral Shigeyoshi Inouye), that Japanese were not assured of air superiority in the region. It was not until early May, when Admiral Inouye had three carriers for operations, that the invasion could begin. On May 7-8, the first carrier battle of the war took place in the Coral Sea. Each side had a carrier damaged, while the American lost the carrier USS ''Lexington'' and the Japanese lost the light carrier ''Shoho''. More important the Japanese broke off their invasion attempt. Significantly, American cryptanalysts had provided crucial order of battle and operational communications intelligence to the Allied commanders in the South Pacific.