i commonly see manga pull this nonsense and it makes no sense.
first of all humans are never a unified block the idea of have a human vrs non-human war where humanity would all be on the humans side is absurd. LOTR exemplified this, rohan didnt put their differences with gondor aside until a legion of orcs started knocking on a few doors, the easterlings sided with mordor, and if i remember right there is another human faction north of the shire that plays no part in the events of LOTR
second of all you really think no-humans would be unified in any way? you think dragons wouldnt have their own self-righteous racial superiority views? what if theres an angel or demon race? and high elven or dark elven one? a war of purists vrs non purists would have many sides
and finally you think there wouldnt be races completely uninvolved in the war? "we live in the water you can fuck off" -mermaids as a neutral party to everyone makes complete sense. dwarves and harpies can also fit this general mold due to liveing underground on in the air respectively