Austria-Hungary and Germany saw an opportunity to conquer and took it

The argument

The assassination of Archduke Franz Ferdinand should not be written off as an unfortunate incident whose consequences were wildly disproportionate to it. Treating the war that way makes the assassination the cause and the powers involved little more than bystanders swept along by events. The argument here reverses that: it was clear that the drift growing between European powers led the duo of Austria-Hungary and Germany to go on the offensive, using the assassination as a guise. On this reading the killing supplied the occasion and the justification, but the intent to act was already in place and would have found some other occasion had this one not arisen. What produced that intent was a change in relative position. The balance of power in the European theater was veering away from Austria and Germany, which put both into a state of policy where they would need to restate their own dominance in an effort of self-preservation. A power that is losing ground faces a choice about timing, and the logic of self-preservation pushes towards acting while the advantage still exists rather than waiting until it has gone. That is what turns a decline into a reason for war rather than a reason for caution. The Austrian position gave this a specific direction. The Austrians were already threatened by Serbian ambition in the Balkans and had felt a need to secure themselves in that sphere before the assassination happened. The killing then raised the threat of military invasion, backing Austria and Germany further into their corner of the continent and making the case for acting first more urgent still. Both powers saw an opportunity to conquer and took it, which is why responsibility for the war rests with them rather than with the man who fired the shot.

Premises

Counter-arguments

Austria-Hungary was Germany's only ally. They flanked Austria-Hungary with a goal of self-preservation, considering the rest of the actors in the theater viewed them as a threat to be removed. Due to this necessity for self-preservation, Austria and Germany should not be viewed as the instigators of the conflict since they were acting purely in response to tensions formed by other powers.

Rejecting the premises

[Rejecting P1] Treating the assassination as a "guise" attributes a prior intention to conquer that the premise does not evidence; a state reacting opportunistically to a crisis it did not create is differently placed from one that engineered a pretext, and the argument treats the two as equivalent. [Rejecting P2] A shifting balance of power and Serbian ambition in the Balkans are the counter's own material for describing both powers as acting defensively, so the same facts underdetermine whether "conquest" or "self-preservation" is the right description — and the other powers whose pressure produced that drift go unexamined.