Key battles in WW1/WW2

Marco Polo Bridge incident...

The Marco polo bridge incident was believed to be the reason why Japan begun its invasion of China and all over east asian countries. But still to this day even the most expert historians still cannot conclude why the Marco polo bridge incident happened in the first place. And why Japan took as a threat to its military and morality. And this would see the beginning of the Second Sino-Japanese war of the 20th century.
But first lets start off with some background...

The Republic of China in the early 1920's was an incredibly unstable and economically fragile nation that had just spiraled out of the collapse of the Qing dynasty in 1912 and would result in multiple chinese states all over. And the most powerful one being the nationalists military. Then would come forth the communists also known as the CCP. And the civil war would begin in the 1930's with the civil war coming to a stalemate rather quickly. However by 1931, the Japanese saw the most politically unpredictable and unstable Chinese republic they had seen since the 1600's. And taking advantage of the situation decided the invade the North eastern province of Manchuria which then they turned into a puppet state and made its own army and puppet government. However the Chinese troops were too weak to resist and they were too focused on the civil war at the time. And the public of the nationalists military were enraged by Japanese forces and their ferocious aggression.

Then in 1937 after the long civil war lasting to mostly a stalemate and then when the Chinese troops and Japanese troops had a border line used which was the Manchurian river. And there was many bridges over the river the main one being Marco polo. And after a series or misunderstandings and accusations, the Japanese and Chinese troops exchanged fire onto eachother sending shockwaves through Asia and the world. Which the Japanese saw the Chinese as the aggressor and the Chinese vice versa. And would give an excuse to invade once again. Finally the Communists and Nationalists put their differences aside to fight their common enemy. And so saw the beginning of the second deadliest front of WW2. The Second Sino-Japanese war had begun...