I just did a Japan campaign and I only managed to take Korea, one Chinese province and Brunei
And then around 1925 everybody ganged up on, forcing me to pay repairments which completely broke my country. I also couldn't field an army meaning Japan fell to a Commie revolution, followed Anarcho-Liberal revolution, then a Jacobin, immediately followed by another Communist revolution, and then another Jacobin turning my nation into the democracy I ended with. I freed Korea in the hope that making myself less powerful would stop nations from attacking me, but eventually Korea itself attacked me.