The Japanese had signed an alliance with the British in 1902, stemming from Britain's lack of participation in the 1895 "Triple Intervention" wherein France, Germany, and Russia settled the occupation of the Chinese Liaodong penninsula by Japanese forces.

In the context of WWI Japan agreed in the opening stages with Britain to sieze Germany's pacific holdings. But more importantly, Japan wished to use the conflict to expand it's influence in China, as the European powers with their military might (which had been used to put pressure on Japanese goals, such as in the Triple Intervention) would be occupied with the European conflict.

The Japanese responded to British requests for naval reinforcement in the Mediterranian, and conducted around 350 escort missions with their destroyer forces. And they helped suppress the 1915 Singapore mutiny against the British government.

Part of the breakdown of the Anglo-Japanese alliance was the Japan's growing ambitions in the far east. As they sought territory their claims encroached on Imperial trade networks and threatened the security of British territorial interests. Furthermore, while Japan had been on the victorious side of WWI, she was not granted much representation in the treaty ending the conflict. If I recall correctly their representives demands were rebuffed so badly, even by their own allies, that they withdrew from the discussion altogether.

---

Dickinson, Frederick R. War and National Reinvention: Japan in the Great War, 1914-1919. Cambridge, Mass.: Harvard U Asia Center :, 1999. Print.

Mutsu, Munemitsu, and Gordon Mark Berger. Kenkenroku: A Diplomatic Record of the Sino-Japanese War, 1894-95. Princeton, N.J.: Princeton UP ;, 1982. Print.

Nish, Ian. The Anglo-Japanese Alliance: The Diplomacy of Two Island Empires, 1894-1907. London: Athlone, 1966. Print.

Sho, Kuwajima. Indian Mutiny in Singapore, 1915. Calcutta: Ratna Prakashan, 1991. Print.