After its initial attacks on Pearl Harbor and Allied interests throughout the Pacific, the Japanese navy continued to expand its conquests over the coming months. On February 15, 1942, Japanese forces took Singapore, which was a very humiliating defeat for Britain. On March 9, after a series of extended sea battles, the Dutch colony of Java surrendered. On April 9, the U.S. territory of the Philippines also fell to Japan. Island colonies, territories, and nations in Southeast Asia continued to fall one after the other as Japanese forces exploded across the South China Sea and into the Bay of Bengal, threatening Burma and even India.
On April 18, 1942, U.S. forces launched a daring air raid to demonstrate that Japan itself was susceptible to Allied attack. Lieutenant Colonel James Doolittle led the ingenious campaign, which originated from the aircraft carrier USS Hornet. Although aircraft carriers were designed to launch fighters, not bombers, Doolittle specially prepared a squadron of sixteen B-25 bombers to fly from the Hornet. The bombers were stripped of all equipment and parts not absolutely necessary for the flight and loaded on board the Hornet with a minimum cargo of bombs.
The lightweight planes managed to take off from the Hornet and fly more than 800 miles to Japan, where they dropped bombs on oil reservoirs and naval facilities in Tokyo and several other cities. The planes then continued on to China to land. Low on fuel, all sixteen planes crash-landed, but two went astray into Japanese-held territory and another landed in Vladivostok, in the eastern USSR. Although the raid did minimal damage to Japan, it was a powerful psychological victory for the United States and demonstrated that the Japanese homeland was indeed vulnerable.
By late spring 1942, Japan had captured most of Southeast Asia and turned its attention southward. In early May, Japanese invasion fleets were ordered to take over Tulagi in the Solomon Islands and Port Moresby on New Guinea—the location of a major Allied base and the last Allied outpost standing between the Japanese navy and Australia. U.S. forces in the area were alerted in advance because of intercepted Japanese radio transmissions. Two American aircraft carriers (the USS Lexington and USS Yorktown), along with several cruisers and destroyers, were dispatched to stop the attacks and protect Port Moresby. The Japanese landed at Tulagi on May 3, before American ships could arrive on the scene. The next day, planes from the Lexington attacked the Japanese forces on the ground at Tulagi and then turned south to join the Yorktown in defending Port Moresby.
The Americans and Japanese finally engaged each other on May 7 in the Battle of the Coral Sea. The entire battle was carried out by carrier-based aircraft, without any ships exchanging shots—the first time in history that a naval battle was waged exclusively from the air. Both sides suffered heavy losses, and the Lexington was sunk. While material losses were comparable for each side, the Allied forces succeeded in their central goal of protecting Port Moresby.
Following the humiliation of the Doolittle Raid and the failure to take Port Moresby during the Battle of the Coral Sea, Japanese strategists knew that something had to be done to eliminate the threat from U.S. aircraft carriers. Admiral Yamamoto Isoroku, who had planned the Pearl Harbor attack, was again put in charge.