This might be a Western bias. I'm sure the Koreans, Chinese, and the many other Asian/Pacific nations would immediately think of the atrocities done by the Japanese.

It bothers me a lot even as a Japanese national that we only ever make media about how the "big bad Americans" came and bombed the shit out of us. Yeah, stories like the Grave of the Fireflies are amazing—it teaches us about children who are affected... in Japan. Do we seriously not have the balls to own up to our past and make movies calling out the horrifying acts we've done to our neighbours?

Mostly the first one (different war), but the lines are not always clear.

Many people cite the 1939 invasion of Poland as an event which kicked off WWII. But far fewer consider the 1931 Japanese invasion of Manchuria to be a WWII event. This is usually labeled as an event of the first Sino-Japanese war.

The confusing part is that the Japanese were basically in a war of some kind from at least 1931 until 1945. The European and American powers were basically uninvolved in any of it, until the attack on Pearl Harbor brought them in.

But the question really is: did Pearl Harbor bring the American and European powers into Japan's existing war, or did Japan join the war started by the invasion of Poland?

As other commenters have said, largely because of Eurocentrism the earlier wars are just ignored for labelling purposes, and Japan is considered to have joined WWII when they attacked Pearl Harbor.

And, in terms of interactions between Imperial Japan and Nazi Germany, they were mostly superficial. If you've thought it never made sense how two countries operating largely on racism could get together and form the "Axis Powers", you're right: that was just another label to unite the Allies against what they saw to be common enemies.

Tldr: Japan was ultimately in their own war which continued into the WWII dates; European/American bias suggests they joined WWII when attacking Pearl Harbor.

The former. Hirohito and Hilter were barely allies. For simplicity's sake you can bundle them but you tend to discount the horrors happening im east Asia at the time. Most people don't care as much about tragedy that happens far away.

It's eurocentrism that basically ignores the Pacific theater. It was also mostly a 1v1 between the US and Japan as far as major powers, the Soviets had some minor skirmishes. Another reason it might be talked about less is there were less survivors, the fighting was brutal on islands and the Japanese used a lot of suicide tactics, there weren't many people that actually experienced the whole war in the Pacific.

The Asian theatre is mostly ignored by the occidental media. Japan commit war crimes against China and Korea, they had ethnic cleansing policies in their « golden circle ». China lost the most men in all the WW2

While the Axis did some minor coordination, the Allied powers definitely coordinated on the strategy of handling both wars at once. There was an agreement to prioritize Germany over Japan as Germany was the more immediate existential threat to the Allies. These agreements included American and British commitments to opening additional fronts in Europe and Soviet commitments to opening additional fronts to Japan.

That said, the was some separation of the Axis. For instance, while Germany declared war on the USA after Pearl Harbor, Japan did not declare war on the Soviet Union following the German invasion of the Soviet Union due to a peace treaty between Japan and the Soviet Union. This is important as Stalin used his Far East troops as part of the Soviet counterattack of the German invasion. Japan hoped this peace treaty with the Soviet Union could be used to negotiate a conditional surrender. This didn't work; the Soviet Union declared war on Japan between the dropping of the two atomic bombs.