Short answers, pulled from the story.
The Second Sino-Japanese War began after Private Shimura Kikujiro of the Japanese China Garrison Army vanished during a night exercise near the Marco Polo Bridge outside Beijing. The Japanese demanded entry into the walled city of Wanping to search for him, but the Chinese garrison of the 29th Army refused, leading to a shot being fired and the subsequent escalation of the conflict.
Japanese forces entered the city of Nanjing on the 13th of December 1937 and committed massive war atrocities known as the Nanjing Massacre. Over the following weeks, troops perpetrated mass executions and tens of thousands of rapes while looting and burning the city, destroying more than a third of the buildings.
The Chinese Communist Party adopted a strategy of guerrilla warfare to preserve strength after retreating on the Long March to Yan'an. The Eighth Route Army carried out operations and established bases in the Shanxi-Chahar-Hebei Border Region while the Japanese military attempted to destroy the economy through systematic violence and blockade tactics.
The Three Alls Policy meant Kill all, loot all, and burn all, and was implemented by the Japanese military to decrease the Communists' human and material resources. This policy involved massacres, slavery, deportations, mass rape, and the use of poison gas and forced starvation against the rural countryside.
The United States increased aid to China under the Lend-Lease Act and provided fighter aircraft through the Flying Tigers commanded by Claire Lee Chennault. The Soviet Union provided the greatest material help from 1937 into 1941 through the Sino-Soviet Treaty and Operation Zet, which included Soviet volunteer combat aviators.
Japan formally surrendered on the 2nd of September 1945 following the atomic bombings of Hiroshima and Nagasaki and the Soviet invasion of Manchuria. The war resulted in the deaths of around 20 million people and led to China becoming one of the Big Four Allied powers and a permanent member of the United Nations Security Council.