The obstacles to an Imperial German conquest of Japan

Could Imperial Germany conquer Japan during its colonial expansion before the Anglo-Japanese Alliance?

Probably not: Japan was already becoming too capable, and Germany would face enormous expeditionary, supply, and occupation demands.

By the Boxer Rebellion era, when Germany might assemble an expedition large enough to attempt conquest, Japan was already rearming and organising. The initial judgement is that Japan would probably be too capable for Germany to defeat at that distance. The attempt would consume resources otherwise needed in Europe and cause heavy losses. Even a successful landing would leave Germany trying to sustain an occupation on the other side of the world against a hostile population unimpressed by European technology.