"The 21 Demands" - Japan demands concessions from China. Historically took place in July 1915.
First option would be Japan takes control of Shantung and southern Manchuria, plus gains military access to China.
Second option (non-historical): as above plus China becomes a puppet of Japan; but balanced by Japan moving much closer in alignment to the Central Powers, plus unrest, perhaps a resource loss (due to British and American sanctions).
China should get a matching event: accept the demands (increases unrest)(historical option) or war with Japan.
Chinese civil wars:
There was almost one in June 1916: President Yuan Shih-kai (powerbase: Peking) versus the Kuomintang (powerbase: Canton) under Li Yuan-hung. Averted by a compromise which resulted in surrending lots of power to the local provincial governors (the warlords).
A second crisis in June 1917, over the question of war with Germany. Tuan Chi-jui, successor to Yuan (who had died in 1916) was pro-war, the Kuomintang was anti. Chang Hsun, a pro-Kuomintang warlord, launched a coup, seizing Peking and putting Emperor Pu-yi back on the throne. Government forces under Tuan crushed the rebellion and restored the republic. China declared war on Germany soon after (14 August).
However, after this time the Peking government never really controlled the south of China. You could even maybe trigger Nationalist China as a revolter against the Republic of China (using the ChiCom tags - the Chinese Communist Party was allied to the Kuomintang in this period). Eventually (1928) the southern faction would conquer the north and re-unify China (kinda) only for the ChiComs to split away themselves.
Stephen