Final answer:
The attack on Pearl Harbor triggered the shift from American isolationism to interventionism, as it united the public's sentiment towards entering World War II. President Roosevelt's declaration of war, approved by Congress, marked the official end of isolationism and the beginning of America's full-scale involvement in the war.
Step-by-step explanation:
The attack on Pearl Harbor was the crucial event that marked the end of American isolationism. Prior to this, the United States had tried to maintain an isolationist stance, avoiding involvement in World War II and focusing on domestic issues. However, the surprise assault by the Japanese on December 7, 1941, on the U.S. naval base at Pearl Harbor in Hawaii led to a dramatic shift in public and political opinion.
Following the attack, President Franklin D. Roosevelt called December 7 "a date which will live in infamy" and requested a declaration of war from Congress, which was approved promptly. The American people, who had been largely reluctant to engage in global conflict, rallied around the war effort. The initial feelings of indignation and incredulity at the attack by Japan soon turned to a unified national effort. Americans from all walks of life and former isolationists joined the military or contributed to the war on the home front as the United States became what Roosevelt termed the 'Arsenal of Democracy'.
The change in the American mindset was solidified when Germany and Italy declared war on the United States as part of their alliance with Japan. This thrust the U.S. into a global conflict against the Axis powers, thus turning two previously separate conflicts into a single world war. The Japanese had miscalculated the American response, hoping that the attack on Pearl Harbor would demoralize the U.S and prevent it from interfering with Japan's expansionism. Instead, it galvanized the U.S to fully engage in World War II, accelerating its transition from isolationism to interventionism.