The growth of the power of Athens, and the alarm which this inspired in Sparta, made war inevitable.

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

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)

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

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.

It was necessary, we felt, to thoroughly terrify our opponents, so that even in hollow victory, they would learn to fear every sunrise ...

...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.

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

Fear makes come true that which one is afraid of.

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.

that shock of fear that runs right through an army before battle, that shock is Dionysos.

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

We dread war; but we follow Washington and Lincoln in dreading some things worse than war. Therefore we desire to prepare against war.

Arm yourself, my heart: the thing that you must do is fearful, yet inevitable.