A little correction: China was not taken over by the Japanese in WWII. Imperial Japan was the one that surrendered to the Republic of China in 1945.


After the Battle of Wuhan where Imperial Japan army lost 140,000 men, basically, the Sino-Japanese war became a draw.


China resisted from 1937 to 1945, to the end of the WW2 when the Japanese finally surrendered.
A war is different from a battle. And the Republic of China won the war.


Let me explain why Imperial Japan could occupy a large part of China rapidly in the beginning of the war.


Simply speaking, three points:


First, an agricultural country couldn’t defeat an industrial country. It becomes more clear when we compare two countries’ steel output in 1934 and 1937. China just began its industry building in 1930s but Japan had already finished it decades before.


Chinese steel output in 1934: 12,048 tons.
Japanese steel output in 1934: 3,840,000 tons.
Chinese steel output in 1937: 550,000 tons.
Japanese steel output in 1937: 5,800,000 tons.


Second, China was in a ten years’ long civil war before 1937.
Chinese Civil War (1927–1936, 1946–1950)
The Kuomingtang (KMT) were fighting against the Chinese Communist Party’s (CCP) army before the Japanese invasion. Even though they announced that they were “resisting Imperial Japan together” in 1936, it still launched several surprise attacks against the CCP’s army during the WW2, like the New Fourth Army incident.


Third, China was not a unified country at that time.
The Japanese Empire was not just an island at that time. Actually, its real size was not even smaller than the actual area that KMT controlled, considering Northern China was still controlled by warlords at that time. These warlords only obeyed KMT’s order superficially.


Somebody said in my comment area: Imperial Japan was not industrialized!
Yes, of course that’s true if considering that there was still a huge gap between Imperial Japan and America or European powers like Germany at that time.
But as I mentioned above, the steel output of Imperial Japan from 1930 to 1940 was 1000 times bigger than the Republic of China’s (in fact, Chinese steel output before 1934 was nearly zero).
Imperial Japan’s tanks were weak but even in that situation, the Chinese didn’t have a reliable way to destroy them.
So clearly, to Republic of China at that time, it was not only just an industrialized but also a deadly and unstoppable enemy.