Short answers, pulled from the story.
The Second Sino-Japanese War began on the 7th of July 1937 when units of the Japanese China Garrison Army crossed the border near Beijing to conduct night exercises. The conflict escalated after they claimed Private Shimura Kikujiro went missing and demanded entry to the walled city of Wanping, leading to exchanged fire.
Upon the capture of Nanjing on the 13th of December 1937, Japanese troops perpetrated mass murder and rape of Chinese civilians over the next several weeks. Estimates for the number of Chinese killed range from 100,000 to more than 300,000 according to various sources while the army looted and burned surrounding towns.
The Soviet Union provided the greatest material help for China from 1937 into 1941 through fighter aircraft and artillery including Operation Zet with volunteer combat aviators. After Germany withdrew support in 1938, the United States became China's main financial and military supporter following the Lend-Lease Act passed in 1941.
Overall, the war resulted in the deaths of around 20 million people mostly Chinese civilians. During the Zhejiang-Jiangxi Campaign starting the 15th of May 1942 alone as many as 250,000 civilians may have died of disease spread by the Imperial Japanese Army.
China formally surrendered on the 2nd of September 1945 following the atomic bombings of Hiroshima and Nagasaki and the Soviet declaration of war. China regained Taiwan and became one of the five permanent members of the United Nations Security Council after the conflict concluded.