Kaiser Wilhelm gave the ‘blank check’ assuming Russia wouldn’t fight. That was his biggest mistake, and his biggest contribution to the catastrophe that was WWI. And when he finally got around to reading Serbia’s response to the Austria-Hungarian ultimatum, he thought it was satisfactory and a “victory” for A-H, without the necessity to go to war. But by then, it was too late.
Sorry, just sleepless…but as has been said before, on 25th of July Serbia acceded with a conciliatory answer to Austrian demands from the 23rd. Strangely, in Vienna it was not telegraphed to Berlin, but sent by mail. When Wilhelm II read it on the 28th, he cabled to Vienna: „Thus, every reason to go to war has been nullified“.
But it was too late.
In 1812, a similar thing had occurred: on June 16th, Lord Castlereagh, the British foreign secretary, announced to the American emissary, Jonathan Russell, that the British government would stop the forceful recruitment of American sailors for the Royal Navy. Russell‘s letters to DC reached the American government weeks later, when hostilities had already begun.
Tragic indeed.
His mistake to not make the promise conditional or to exercise oversight over the Austro-Hungarians in part by cautioning them to not provoke anything. Additionally, not doing his due diligence on just what he was writing a blank check to.