That depends on the historian. I've the Japanese invasion of Manchuria referred to as the start of WWII several times, and I personally believe it is.

Those that argue that the invasion of Poland instead is the start argue on several reasons, a few of which are as follows:

The Second Sino-Japanese War was mostly between China, Korea, and Japan. Japan was already friendly with Germany, but their only official alliance was against communism. At the time it was mostly a regional conflict that the rest of the world wanted nothing to do with. A lot of people didn't consider it a world war until the Axis and Allies were formed as two global forces against each other.