In this scenario conflict already exists between the powerful and powerless; for the enslaved to rebel is only a continuation of the conflict begun by the powerful.
War is defined here, in the original premise, as the opposite of peace and as a synonym of hostility. Conflict and slaughter would be considered non-peaceful and hostile actions.
There is always justification for a nation defending its sovereignty from an invading force. And for invading there is a chance that the country needs a particular resource and invading is the best way to gain access to that resource.
The 'country' is a construct, an illusion; why should the governments of two peoples fight for a resource when it is in the best interests of the people of both nations to share it as needed?
It may not actually be in the best interest of the construct country's populations. That point applies something generally when it's not always or even largely true.
What is better overall for the populations, splitting the resources so there is barely enough for all, or risking total destruction and loss of the resources in the hopes of denying that life to the other?