It depends on your definition of the beginning of the war. Japan had already been at war with China since 1937 which is before the traditional starting date of World War two which is September 1939 the German invasion of Poland. The key difference between the Japanese invasion of China and the Nazi invasion of Poland in gauging the start date of the war is that the invasion of China didn't escalate into a conflict with other powers like the invasion of Poland did, at least not directly or right away. This is the same reason why Nazi Germany seizing power in Czechoslovakia or Austria is not considered the beginning of the war.

As for Japan's foreign policy during the war well its complicated. See the idea of Japanese power overseas is very caught up with their security. Internally they viewed the reason why Western Powers were so secure was their ability to project power overseas in the form of colonies. So their invasion of Korea, China, Taiwan and later South East Asia was all done in forming what they dubbed a Greater East Asia Co-Prosperity Sphere. This was Asian powers that were free of western influence but beholden to Japan (So basically like any other over seas imperial empire except with Japan and not a European power)

Now the war with China is largely what caused the escalation in conflict with America and the British Empire. Basically the invasion of China was a very brutal and long conflict and these brutalities caused a stir internationally. In response to Japanese atrocities America broke off trading with Japan. This was quite damaging to the Japanese war effort as American oil was critical. To counter this the Japanese expanded their war front into South East Asia and the Western Powers that had a presence there (Britain, US and the Netherlands.) This was to gain more resources and hopefully force US to terms to open up trade once again.