After securing strategically located bases during its war with China, Japan set out to create its long-coveted greater east Asia co-prosperity empire. Opening with a crushing attack upon Pearl Harbor on 7 December 1941 that temporarily neutralized the U.S. Pacific Fleet, the Imperial High Command quickly followed by dispatching large forces to seize the Philippines, Malaya, and the Netherlands East Indies and preparing plans for new bases from which to strike Australia and India. By June 1942 Japanese authority on the Asian mainland had extended beyond Malaya into Thailand and Burma. In the western Pacific, it encompassed most of the larger islands north of Australia and east of Midway.
In the wake of such astounding military success, Japan decided to push onward rather than consolidate its gains. Its next objectives, New Guinea and the Solomon Islands, were clearly to be used as steppingstones to Australia. Between those objectives and the Australian continent was the Coral Sea, where in early May the American Navy had checked a powerful Japanese fleet in a battle that frustrated the enemy's hope for an early invasion of Australia.
Remaining on the defensive throughout the Pacific, the United States hurriedly fortified island bases along a great arc extending from Pearl Harbor to Sydney to keep open the shipping routes to Australia. With only limited numbers of troops available, it nevertheless joined Australia in planning an offensive in New Guinea and the Solomons to halt Japanese advances. To command this offensive in what became known as the Southwest Pacific Area, President Franklin D. Roosevelt selected General Douglas MacArthur, leaving the remainder of the Pacific theater under the direction of the Commander in Chief, U.S. Pacific Fleet, Admiral Chester W. Nimitz.
Nimitz's command was divided into three combat areas (north, central, and south). The North Pacific Area extended west from the continental United States, Canada, and the Territory of Alaska across the Pacific to the Asian mainland. Included within Nimitz's North Pacific Area were Japan's northern islands, the Kuriles, and, just 650 miles to the east, Alaska's Aleutian chain.
Protruding in a long, sweeping curve for more than a thousand miles westward from the tip of the Alaskan Peninsula, the Aleutians provided a natural avenue of approach between the two countries. Forbidding weather and desolate terrain, however, made this approach militarily undesirable. While spared the arctic climate of the Alaskan mainland to the north, the Aleutians are constantly swept by cold winds and often engulfed in dense fog. The weather becomes progressively worse in the western part of the chain, but all the islands are marked by craggy mountains and scant vegetation. Despite such inhospitable conditions, neither the United States nor Japan could afford to assume that the other would reject the Aleutians as an impractical invasion route.
Japanese concern for the defense of the northern Pacific increased when sixteen U.S. B-25 bombers, led by Lt. Col. James H. Doolittle, took off from the carrier Hornet and bombed Tokyo on 18 April 1942. Unsure of where the American raid originated, but suspicious that it could have been from a secret base in the western Aleutians, the Imperial High Command began to take an active interest in capturing the island chain.
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