Michelle

Yes I believe war is inevitable. Even if democratic peace theory holds, there are plenty of resources and scarcity of them, mainly water, that would make it beneficial or certain states to go war. Even if all states move towards democracy, there is the question of what exactly counts as democracy because if illiberal democracies such as Russia are included then there is a chance that these illiberal democracies will war with each other.
Also, the fear factor needs to be taken into consideration. Since it is highly unlikely that all states will cooperate with each other and share accurate and full information a state, out of fear, can preemptively strike. Even if the recipient was not intending full out war the preemptive strike will trigger it.
I also think the resource that will be fought over the most is water since its scarcity is growing due to pollution and as long as this is the case war will be fought over this necessity.