In the UN's Rome Statute of the International Criminal Court you'll find the definition of genocide,
For the purpose of this Statute, "genocide" means any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such: (a) Killing members of the group; [...]
It makes sense if you're trying "with intent to destroy [...] in part a ethnic, racial, or religious" group to utilize the term "genocide." But if one nation state's army declares war on another nation state's army and enters into open hostilities both will be actively trying to destroy at least a part of the other's national group (the enlisted armed combatants if nothing else).
Does that mean every war is genocide?