It was the rise of Athens and the fear that this instilled in Sparta that made war inevitable.

All these calamities fell upon Hellas simultaneously with the war, which began when the Athenians and Peloponnesians violated the thirty years' truce concluded by them after the recapture of Euboea.
Why they broke it and what were the grounds of quarrel I will first set forth, that in time to come no man may be at a loss to know what was the origin of this great war.

The real though unavowed cause I believe to have been the growth of the Athenian power, which terrified the Lacedaemonians and forced them into war; but the reasons publicly alleged on either side were as follows.

(Book 1 Chapter 23.3-6)

As long as there are sovereign nations possessing great power, war is inevitable.

Yes, a war is inevitable. Firstly, there's you fellows who can't be trusted. And then there's the multitude who mean to have bathrooms and white enamel. Millions of them; all over the world. Not merely here. And there aren't enough bathrooms and white enamel in the world to go round.

To come under siege, he decided, was the inevitable fate of power.

The Great War was born in anger. The war rising around us will come in hatred wrapped in a flag, with a madman at the helm. And that is the war I fear most

The means of defence against foreign danger have been always the instruments of tyranny at home. Among the Romans it was a standing maxim to excite a war, whenever a revolt was apprehended. Throughout all Europe, the armies kept up under the pretext of defending, have enslaved the people.

As long as armies exist, any serious quarrel will lead to war.

...the consciousness of being at war, and therefore in danger, makes the handing-over of all power to a small caste seem the natural, unavoidable condition of survival.

Of all the enemies to public liberty, war is perhaps the most to be dreaded because it comprises and develops the germ of every other.

War was always here. Before man was, war waited for him. The ultimate trade awaiting its ultimate practitioner.

The city had the air and movement of hysteria, and the citizens were crying, in every accent of anger and alarm, that the new forces must at any cost be brought under control. Prosperity never before imagined, power never yet wielded by man, speed never reached by anything but a meteor, had made the world irritable, nervous, querulous, unreasonable and afraid.

Europe is overpopulated, the world will soon be in the same condition, and if the self-reproduction of man is not rationalized... we shall have war.

Fear begets fear. Power begets power. I willed myself to beget power.