Well, WWII in the Pacific started when China and Korea were invaded by Japan in 1937, followed by them taking British islands, well before they attacked Pearl Harbor.

Later, the US had fighter pilots officially attacking the Japs in China right after Pearl Harbor, like two weeks later (the Flying Tigers) showing to me at least, that we were likely involved helping China prior to Pearl Harbor.

The US cut off oil and metal exports to Japan, which needed them as part of their war effort. Supposedly that provoked Pearl Harbor, but I suspect that there's way more, especially since I believe that FDR knew it was coming and had the aircraft carriers pulled out from the naval base.

But as for Japanese atrocities, there are tons and tons of personal accounts, pictures, survivor tales, etc. etc. Japan refused to accept cultural guilt for it, which is great, and I wish Germany had done the same, but they Japs did some nasty, evil things. Look into Unit 731. The US took all that data as part of our nascent biological weapons programs as we started the Cold War.

https://www.grunge.com/232155/inside-japans-horrifying-wwii-biological-warfare-project/