Japan had an important contribution to the war, but more in an economic and substitution area of it. As Japan had signed a alliance with the UK in 1902 it smoothly entered the war in August 1914. Despite this, this was hardly an alliance which greatly benefitted the UK. Even in 1914 Japan was beginning a slippery slope towards its eventual international position in the 1930s, and the war would spur this on.

Its most obvious military contributions were twofold; firstly, they helped the British seize German colonies in China and the Pacific. The Japanese were one of the prime movers in the landing at Qingdao/Tsingtao, the only German leasing in China from the Boxer Rebellion. This was near the old battleground port of Port Arthur/Dalian which the Japanese so fervently fought over with the Russians in 1905. Perhaps this illuminates the expansionist glint which existed in the government's eye at that time; visions of rapid, combative expansion into China from Japanese Korea were first formulated in this joint British/Japanese campaign, but ultimately it was the only major Japanese military expedition of the war.

Their second major military contribution also has another two layers. Before the war, the Commonwealth of Australia had agreed with the UK that Australian troops would seize all German Pacific colonies in the case of war. This was due to their distrust of Japan/the Japanese (both politically and racially motivated) and the idea of the Australian sphere of influence. With the sudden demands of war, though, and Japan soon found itself eagerly occupying all of these small islands throughout the North Pacific due to the Australian inability to sacrafice reinforcements from Gallipoli (even though German New Guinea was seized in September 1914).

So, here too, did the Japanese take some of the pressure of the British shoulders. They supplanted the need of the British to provide troops for the landing at Qingdao, and for British warships to land marines at all these Pacific islands, which could house and distract German raiders.

And, for the most important and beneficial, the issue of German raiding ships. In both wars, German commerce raiders were deadly in their operation along Imperial shipping lanes to and from the Australia station. Tens of thousands of tonnes were sunk; as such, and with Admiralty requests, the Japanese navy patrolled most of the Pacific sea routes and was instrumental in suppressing German raiders. They were so succesful at this that the British even requested a small Japanese squadron for operation in the Mediterranean from 1917 onwards.

Economically, too, they supported the Allies. Not only did they provide shipping for troops to the Western Front, but munitions and the likes. My best example is for their popular providing of trench mortars. Trench mortars were a new invention of 1915, and a German one at that. They were deadly in their use on the Western Front. But it was at Gallipoli, where the Japanese provided four trench mortars for the ANZACs in the first month, that most likely helped hold the ridges along the beach during the first few weeks.

By Versailles in 1919, the Japanese had a representative sitting in on the peace talks, had been given formal control over the Northern Pacific islands and had made influential inroads into Northern China.

But, as I said before, the seeds of the destruction for this alliance had long been growing from the mistrust of Western powers of Japanese ambitions. The United States had been vying for control of the Pacific as well - with Phillipines/Guam and Hawaii, it was an important arena - and Britain felt threatened for its eastern colonies and dominions. The tensions of this eventually snapped, breaking the alliance in 1922 with London formal approval and Japanese popular approval. The largely ignored Washington Naval Treaty of 1922, to Tokyo, restricted them from the goals they largely grasped at and pursued during the war.

Their post-war conduct and unfortunate separating directions of British and Japanese foreign policy should not betray the professional usefulness they provided each other. The Japanese took immense pressure off the British Empire in its upholding of the Eastern theatre (if anything performing the opposite of their second war role) while the British were magnificent enablers for the Japanese expansion into China and Pacific.

[My primary source here is the ever reliable C.E.W Bean. Writing the Australian Official War History, he had in his writing an uncritical and fair observation on all aspects of the war.]

Prior to the First World War, Japan and Great Britain shared an alliance which is mostly the reason they joined the on the side of the allies. Japan offered to join the war in the hopes of gaining the German territories in the pacific and the allies where happy to have the added support in that theater.

As for her involvement in the war, it was almost entirely naval. German ships paroling pacific and disrupting trade in the area was a serious threat to allied supply lines, and Japan's involvement helped nullify that threat and where successful in their encounters with the navies of Germany and Austria. As for any ground involvement with any central power forces, there are none (that I have ever read of). The German colonies of the pacific were taken without much, if any, involvement of their armies. This is not to say that the Japanese did nothing in the European theater, some of Japan's navy aided the allies in the Mediterranean and Japan provided many of the resources that where so desperately needed in Europe toward the end of the war.

The result of Japan's involvement put them on the world stage and earned them major seat in the League of Nations as well as the previously German islands they had taken in the pacific.