It's a Real-Time Strategy game released on the Gamecube and the Wii. It's gameplay differs from Advance Wars obviously but it has a similar charm with it's humor and light-hearted take on war. Also has the same ranking system Advance Wars has (Speed, Power, and Technique).
It is apart of the Wars series even though it's from a different developer and has different branding. "Battalion" I believe was used to make sure consumers didn't buy the game expecting it to be like Advance Wars. As a matter of fact, the initial name for Battalion Wars when it was revealed at E3 2004 was "
Advance Wars: Under Fire" but it got changed to Battalion Wars later.
EDIT: I forgot to mention the fact that the Battalion games carry on the "Famicom Wars" branding in Japan. I guess they didn't bother to distinguish the two.