It concerned a war between what I think were tiny yellow hippos who lived underground. The illustrations were incredibly detailed. I'm about to have a child of my own and would really like to give them that book at some point, but have no clue where to find it. I can't remember the title, but do remember the book's cover had a blue binding.

Hattie Hippo?

I'm looking for that book,too. I have been for several years, mine was over-sized. There were warring generals and amazing flora and fauna details. I think they were tiny hippos, but other garden animals were helping the good guy.It seem like there were cutaways of underground army bases, with every rock,frog skeleton,ant tunnel and worm hole shown.

So was it Hattie Hippo?:
hattie hippo book - Google Search