On 24 July 1914, the Austro-Hungarian Chief of the General Staff, Franz Conrad von Hötzendorf (pictured), sent a letter to the Austro-Hungarian Minister of Foreign Affairs, Leopold Count Berchtold. In his letter, Hötzendorf infroms Berchtold of military intelligence that suggest the mobilisation of the Serbian Army.
The Imperial and Royal Chief of the General Staff to Count Berchtold. Vienna, 24 July 1914.
I just received a telegram (at 12 p. m.) according to which the mobilisation was proclaimed in Sabac [in Serbia] at 16:00 this afternoon. This news came from the Corps-Commander in Zagreb [in Austria-Hungary] and from the Commander of the Gendarme-Division in Mitrovica.
Servia’s answer to the [Austro-Hungarian] ultimatum appears to be the mobilisation.
This demands a similar measure on our part, and I advise that the mobilisation should be proclaimed immediately, that is on 25 July instead of 26 July. The first day of the mobilisation would be 28 July. I am sending this letter to the War-Minister first.
Your Excellency’s devoted servant
Source: 1919 Austro-Hungarian Red Book with minor edits.