William Lawrence Sr Omnibit 4-29-15
The tiny Balkan nation of Montenegro declared war on Japan in 1904 during Russo-Japanese war. Why? Who knows? Montenegro is about 6,000 miles from Japan and few in Japan had heard of it.
But, hey, it made sense at the time.
Anyway, the declaration remained in effect for 101 years largely because Montenegro was made part of Yugoslavia after World War I, and then was part of Serbia until becoming independent in 2006.
At that point Japan made peace with it.