The 2 men most responsible—July 26

Berchtold

Leopold Anton Johann Sigismund Josef Korsinus Ferdinand Graf Berchtold von und zu Ungarschitz, Frättling und Püllütz (lets call him Berchtold for short), was Austro-Hungarian Imperial Foreign Minister in 1914. He had been accused of indecision during the recent Balkan wars. That wasn't going to happen again.

Conrad

Franz Xaver Joseph Conrad Graf von Hötzendorf (Conrad), was Chief of the General Staff of the Austro-Hungarian Army in 1914. He believed in the inevitability of a struggle between German and Slavic civilization (he was way ahead of his time in some unpleasant ways), and had pushed for a pre-emptive military attack on Serbia twenty-five times in his career.

So Berchtold and Conrad, with the Kaiser behind them saying that they can basically do what they want and will have his unstinting support, are at the centre of the rapidly developing July Crisis. They had not even wanted the fiction of an ultimatum. Their plan to crush Serbia depended on getting it all over quickly, done and dusted before Russia could react. Already a month has gone by and, on July 26, Conrad explains to Berchtold that an invasion of Serbia will not be possible for a number of weeks. 

Oops!