After the surprise attack on Pearl Harbor on December 7, 1941, America’s mobilization to go into action against Japan and Germany was swift, but the process of preparing for full-scale involvement in the war took some time. The U.S. had been building up its military and industry before the attack, but the assault on Pearl Harbor served as the catalyst for rapid and coordinated action.
Within hours of the attack, President Franklin D. Roosevelt addressed the nation, delivering his famous "Day of Infamy" speech, which rallied Americans and called for a declaration of war against Japan. On December 8, 1941, Congress responded by officially declaring war on Japan, with only one dissenting vote. This marked the beginning of America's direct military involvement in the war against Japan.
The initial response was focused on Japan, with the U.S. Navy and Air Force quickly shifting their resources to the Pacific theater. The American military had already been expanding its forces in response to growing global tensions, so within days, reinforcements were sent to the Pacific, including aircraft and personnel for combat operations. The first major American offensive in the Pacific came with the Battle of Midway in June 1942, which was just six months after Pearl Harbor.
Regarding Germany, America’s formal entry into the European theater came shortly after Japan's declaration of war. On December 11, 1941, Germany declared war on the United States, and the U.S. reciprocated with a declaration of war on Germany. While the U.S. had already been supplying Britain and the Soviet Union with aid through the Lend-Lease Act, the first significant American engagement with Germany was the U.S. involvement in the North African Campaign, which began in November 1942—about a year after Pearl Harbor.
In summary, while America immediately declared war and began mobilizing for action after the attack on Pearl Harbor, full-scale military operations against both Japan and Germany took several months to organize and execute. The rapid response in the Pacific contrasted with the more gradual buildup in the European theater, reflecting the scale and complexity of the war effort.