China's role in WWI (1914-19)

Although China doesn't fight directly in WWI [5], its support proves decisive for the further victory of the Allies. When the war broke out, China decided to remain neutral. However, 50,000 troops were secretly offered to the British Minister John Jordan in orden to retake the Chinese province of Qingdao. Jordan rejected the offer and instead, the Japanese used their forces to oust the Germans.

Nevertheless, the massive amount of casualties the British were suffering during the war made Jordan come up with the idea of requesting Chinese aid and support. China would enter the war in the side of the Entente if the members agreed including Japan. Japan sees this as a threat to their power and dominance in the east and refuses to allow them to join. 

Due to the denial to fight directly, Shikai decides to send secret support to the Allies: voluntary non-combatant workers. Their supply includes munition, transport and reparation of tanks.

Finally, the Allies win the war and sign the Treaty of Versailles. China, however, refuses to do so as they expected to receive the mainland territory of Shandong. Instead, it is given to Japan.

*[5] https://www.smithsonianmag.com/history/surprisingly-important-role-china-played-world-war-i-180964532/