If we cannot find a sustainable alternative to maintaining our societies, then yes, war will be inevitable. For the most part, wars were fought to gain access to resources, to expand territory and the sphere of influence of the aggressor.
It make sense that "World War III" would be fought over access to ever-depleting resources, that mankind is still largely dependent upon. It also makes sense that the perpetrators of this war, would be the societies which have made their population fully dependent on an unsustainable way of life, requiring these depleting resources.
I do think such an occurrence can be averted, mainly through a deconstruction of current, unhealthy power constructs which have expanded beyond their original purpose, whose primary objective is no longer to help maintain, but to expand their own power and influence in the manner of a cancerous cell, but I do not see many steps being taken to avert it now. We have grown very dependent on them, more than ever. Dependency makes a man vulnerable, and vulnerability can be exploited, as a means to wage war, or whatever is deemed to be necessary, at the command of those who provide that which the population is dependent upon.