We're used to thinking of Japan as a world power, even a world conqueror. Until 1945, they were a viable military player; after 1945, they've become a global economic powerhouse. Much like the Kingdom of Prussia, therefore, it stands to reason that Japan must have always been this awesome.
As with most National Tropes, this assumption is simple, obvious, and wrong.
It's worth mentioning that while this trope is wrong, it isn't innately so. It isn't impossible to have post-Sengoku Japan keep its borders open, adopt the gun, and turn into a regional power.