Final answer:
America's policy towards Japan shifted from shock and non-intervention after Japanese aggression in Manchuria and China to diplomatic isolation and economic sanctions as aggression continued. This approach evolved into support for Japan's post-WWII reconstruction to counter communism, ultimately leading to a cooperative US-Japanese relationship.
Step-by-step explanation:
After the Japanese invasion of Manchuria and China, America's policy towards Japan underwent significant changes. Initially, the United States responded with shock and condemnation of the violence but maintained a policy of non-intervention, even after Japanese aircraft attacked the USS Panay. However, as the situation worsened and Japan continued its aggressive expansion, the US responded by supporting the Chinese protest at the League of Nations and engaged in diplomatic isolation of Japan, which led to Japan’s withdrawal from the organization. American public sentiment against Japan grew, driven by reports of atrocities such as the Nanjing Massacre and by protests from groups such as Chinese Americans and Protestant churches involved in missionary work in China.
Ultimately, this shift in attitudes and Japan's continuing aggression would lead the US to abandon its neutrality. As Japan focused on building its empire, the United States began to move towards engagement, eventually replacing a stance of withdrawal with direct investment and preferential economic treatment in the post-World War II era. This strategy aimed to both rehabilitate Japan's economy and counter the communist influence in Asia, notably from the Soviet Union and China. The United States eventually guaranteed Japan's security through a treaty, leading to a new, cooperative phase in US-Japanese relations.