There were several events that intensified American animosity towards the Japanese during the first half of the 20th century, including:
The Russo-Japanese War (1904-1905): The war was fought between Russia and Japan over territorial disputes in East Asia. Japan's unexpected victory over the Russians led to increased admiration and respect for Japan in the United States, but it also fueled fears of Japan's growing military power.
Japanese immigration: The arrival of Japanese immigrants to the United States in large numbers in the early 20th century resulted in increased tension between the two countries. Japanese immigrants faced discrimination, particularly in California, where many settled and worked in agriculture.
The Great Kanto Earthquake (1923): The earthquake and resulting fires caused widespread damage and loss of life in Tokyo and surrounding areas. In the aftermath, rumors spread in the United States that the Japanese government was hiding the true extent of the disaster, which further fueled anti-Japanese sentiment.
The Mukden Incident (1931): This was a staged event by the Japanese military to justify the invasion of Manchuria, which was then part of China. The incident led to increased criticism of Japan's military expansionism and aggressive foreign policy.
The attack on Pearl Harbor (1941): The surprise attack by Japanese forces on the U.S. naval base at Pearl Harbor, Hawaii, resulted in over 2,400 American deaths and led to the United States' entry into World War II. The attack further intensified anti-Japanese sentiment in the United States and led to the internment of Japanese Americans during the war.