The Anglo-Japanese Alliance was a strategic alliance between Great Britain and Japan that lasted from 1902 until both powers jointly denounced it in 1922. While the early years of the alliance were relatively peaceful, between 1914 and 1921, Anglo-Japanese relations were marked by numerous conflicts and difficulties against the backdrop of World War I. Ending the alliance was, on its face, a decision that the two nations reached by mutual consent. However, the decision was followed by significant Japanese resentment. Additionally, until July 1921, British leaders still believed that the alliance was a worthwhile endeavor. It was not until The United States issued an informal ultimatum to the British that they abandoned hope of renewing the Anglo-Japanese Alliance and sought a larger treaty including themselves, Japan, the United States, and France. To understand how the British reached their decision to end the alliance, this thesis examines the alliance between 1914 and 1921. During the war years, the British were fractured on whether Japan could be trusted, but after the war’s end, their vulnerable position in the Pacific made clear that Japan was still an important ally. Coinciding with Japan’s rise as an imperial power, the United States also rose to prominence and viewed Japan as a direct threat to their expansion in the Pacific. From 1919 to 1921, the British carefully attempted to avoid irritating the Americans while retaining their alliance with the Japanese. Instead, they were forced to drop the alliance and appease the United States.