After the bombing of Pearl Harbor on 7 December 1941, President Roosevelt declared war on Japan the next day. Three days later, Hitler declared war on the United States. Thus the American military was thrust suddenly into the Second World War, fighting in two different Theaters, against vastly different enemies. General Douglas MacArthur, who had been commanding forces in the Philippines, arrived in Australia in March 1942 and was soon thereafter appointed as the Supreme Commander of all Allied forces in the Southwest Pacific Area (SWPA).

The SWPA was constituted on 18 April 1942 by agreement among the Governments of Australia, the United Kingdom, the Netherlands, and the United States. On that date, General MacArthur assumed command and proceeded to establish his General Headquarters at Melbourne. His Assistant Chief of Staff for Intelligence was Colonel Charles A. Willoughby, who oversaw the complex intelligence network of the SWPA Theater. Willoughby set up a number of Allied intelligence collection organizations under his direct control: The Central Bureau handled Cryptologic functions. Human Intelligence came from the Allied Intelligence Bureau, whose mission was to collect intelligence through clandestine operations behind enemy lines, conduct sabotage operations and recruit aid from the natives. The Allied Geographical Section was formed to collect and assemble topographic information, and to prepare and publish reports and locality studies on areas of immediate tactical interest. The most productive single intelligence agency established under the G2 of the SWPA was the Allied Translator and Interpreter Service , or ATIS, which was organized on 19 September 1942.

ATIS effectively neutralized the Japanese language barrier -- one of the greatest advantages possessed by the Japanese, as it was almost as effective as a secret code. The G2 employed hundreds of second-generation Japanese-Americans (called Nisei) in linguist detachments under the ATIS. They accompanied assault landing forces across the beachhead and on inland, conducted spot interrogations, translated captured maps and plans, and gave the psychological warfare planners excellent insight into the morale problems of enemy soldiers through the exploitation of letters and diaries. Other captured and translated documents revealed the enemy's food and supply problems, his order of battle, the effects of Allied air attacks and the effectiveness of both Allied and Japanese weapons.

The Commander of the ATIS, Colonel Sidney F. Mashbir, in his book I Was an American Spy, called the ATIS document translation process a "brain-power quantity production line." He stated that by the end of the war, ATIS had interrogated 14,000 prisoners, translated almost two million documents, and published more than twenty million pages of Japanese intelligence. The irony was that the Japanese kept scrupulous records, which they rarely encrypted, having absolute confidence in the security the language barrier afforded. ATIS used this to their complete advantage, compiling nearly complete sets of unit papers, including war diaries, organizational rosters, intelligence reports, pay books, postal-savings books, correspondence, and personal possessions. Using these captured items, they slowly and patiently built up a mosaic picture of the enemy force. By the time the American forces reached Manila in January 1945, Mashbir boasted that [ATIS] "literally knew more about the Japanese Army than most of its own officers, because, as a matter of fact, we had their records. This was equally true of the Navy and Air Force." [italics in original] Over the course of the War, ATIS issued thousands of printed documents, providing intelligence of immediate operational importance as well as overall strategic value.