Following the outbreak of hostilities, the British Empire requested assistance from Japan under the terms of the Anglo-Japanese alliance. Although the German presence in the Far East was small, the Kaiserliche Marine controlled Micronesia, German New Guinea, and the Chinese strategic port of Tsingtao, where the German traders built a famous brewery in 1903. The British were happy to let the Japanese do the heavy lifting in China, and since the Japanese were looking to expand their influence over the Pacific Rim, they were happy to oblige. (This expansion of Japanese power with approval from the British was not popular stateside.)
The emperor sent an ultimatum to Germany on August 15, 1914, but it received no response and elapsed on August 23, at which point the Japanese considered themselves to be in a state of war. Full text from the Gettysburg Times article can be found here.
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