I started reading up on WWII recently and it always surprises me that Japan somehow managed to start a full scale land invasion of China. I don't understand the history of this well, but from my modern perspective of what China and Japan are respectively as military powers, If Japan tried to do that they wouldn't just lose, they would get flattened, if China wanted to they could completely demolish Japan, how was this different during WWII?
The key part of your post OP is "my modern perspective of what China and Japan are respectively as military powers"
China and Japan were very different countries in the early 20th century.
Japan was a rising independent Asian power with imperial ambitions. In the Russo-Japanese War of 1904-1905 Japan managed to humiliate Russia and take half of Sakhalin Island. Japan joined the winning side of WWI and afterwards annexed Korea.
China was in much worse position. It was and divided between Communists and Nationalists who were at that time fighting a civil war. There was also a lot of corruption in the Nationalist Chinese government preventing reform and rotting the country from the inside.
However the 1937 invasion was no cakewalk for the Japanese by any means. They took heavy casualties during the invasion. Even when the Nationalist government in Nanking surrendered the war wasn't over. The Japenese fought against Nationalist and Communist Chinese resistance forces until 1945.