Most people believe that WW2 started on September 1st 1939 when Germany and the USSR invaded Poland, but conflict in Asia and the Pacific had been at war for a while.
On the 7th of July, 1937, Japan attacked China. The two countries had been engaged in minor skirmishes since 1931 but it wasn't full out war until The Marco Polo Bridge Incident. The Japanese conducted a military drill which scared the Chinese, who fired shots. The fire was returned and things soon heated up. It wasn't for over 4 and a half hours that things got serious when the Japanese brought in some heavy artillery pieces and a crack squad of machine gunners. In the end, the battle was fairly indecisive, but it opened the door for a major conflict in China, a conflict that until the entrance of the British Commonwealth and America, was very much going in Japan's favour.