Vast unkown battles.

[Cheezy the Wiz]

Off my own bat I'll throw in Khalkin Gol (sp?)

Certainly a disaster... for both sides.

EDIT: Wait, this isn't the military disasters thread!

Yes, it's a huge battle, and no one knows about it, or the rest of that undeclared war between Japan and Russia.

 

[sydhe]

Here are a couple of huge battles from the Russo-Japanese War that pretty much decided nothing:

http://en.wikipedia.org/wiki/Battle_of_Shaho
http://en.wikipedia.org/wiki/Battle_of_Sandepu