Greetings! This is a rather specific question about the curious sequence of events which took place near the end of the July Crisis, and it is one which links to the larger political decisions taken in Austria-Hungary regarding the declaration of war. This response will first deal with the general situation in Vienna in the days leading up to the declaration of war (because there are plenty of confusing chronologies there as well), before then analysing the accuracy of the source OP has brought up regarding a border skirmish on the 27 July. Let’s begin.

The Dual Monarchy Mobilises

‘The responsibility which is resting on all advisers of the Crown in these decisive days makes it my duty to turn with the sincere request to Your Majesty to order mobilization immediately in case of an insufficient answer from Serbia. Given the contents of our note it seems to me there is no other possible way.’

- Hungarian Prime Minister Istavan Tisza to Emperor Franz Joseph I, 25 July 1914.

When the Serbian reply to the Austro-Hungarian ultimatum was received in Vienna, it fulfilled the expectations of the Common Ministerial Council. Although a masterpiece in conciliatory diplomacy and de-escalatory rhetoric, the Serbian reply was deemed ‘unsatisfactory’ by all in Vienna, and thus the next steps in the crisis were now theirs to take. Baron Giesl, the Austro-Hungarian ambassador at Belgrade who had delivered the ultimatum on 23 July, recounted his view of the document upon being given it by the Serbian prime minister himself, Nikola Pasic:

‘Certainly I found several intimations of a conciliatory acceptance - probably the remnants of the original version… but ultimately nearly all our demands were twisted, robbed of their meaning and purpose, and their fulfilment, if not directly refused, was so hedged in reservation [verklausuliert] that it was in praxi useless. The matter was absolutely clear: I had nothing to weigh, nothing to decide, only to state the facts and then, as ordered, to depart.’

It is at this point that we must address a key topic within the question: Austro-Hungarian mobilisation. Because the decision-makers at Vienna had mainly envisioned an Austro-Serbian war as the outcome of the crisis, their military officials had been told to avoid mobilising troops on the border with Russia, lest their presence embolden St. Petersburg to come to Serbia’s aid down the line. As such, the general staff were faced with the problems of enacting a partial mobilisation against only Serbia. Historian David Stevenson elaborates on what this meant in practical terms:

‘Barracks and depots were to be placed under guard, and the frontiers and the railways secured. Partial mobilisation meant that of B-staffel (‘War Case B’) and the Minimalgruppe Balkan, with additional troops to make good casualties and guard the Italian border - in total, eight army corps or two-fifths of the land forces, as well as the whole of the navy and the Danube flotilla. The Galician units were ordered to stay in their quarters, those opposite Romania and Italy to do nothing that might disturb their neighbors, and even those opposite Montenegro to show ‘the greatest restraint’.’

This was a deliberate risk on Austria-Hungary’s part. Their railway networks were not able to accommodate a reversal of the mobilisation order, owing to the lack of transport capacity at key sections of the lines. As such, if Russia were to declare war and a general mobilisation became necessary, the Dual Monarchy could only begin such preparations by the eleventh day of mobilisation (M+11), leaving them dangerously vulnerable to pre-emptive maneuvers by both Serbia and Russia. Now, with regards to the somewhat unclear dates and the delay between the signing and the declaration, there are solid answers to both of them. It is also worth clarifying the somewhat messy mobilisation schedule, as there are usually conflicting dates regarding the order versus the actual beginning of mobilisation. T.G. Otte clarifies the chronology of events well:

‘Orders for the mobilization against Serbia, that is partial mobilization, were issued late [9:23 p.m.] on 25 July. But as the following day was a Sunday, 27 July was fixed as the day on which the measure would be proclaimed, and Tuesday, 28 July as the first day of mobilization.’

On 27 July, Franz Joseph I sanctioned the official declaration of war against Serbia. It was to be issued the following day, at 11:10 a.m., by Foreign Minister von Berchtold, to coincide with the first day of Austro-Hungarian mobilization. This would immediately nullify any subsequent attempts by the Entente Powers to propose mediation on the issue, as an international (and official) state of war would exist between Austria-Hungary and Serbia. For reference, the final telegram expressing this state of war went as follows:

‘The Royal Serbian Government not having answered in a satisfactory manner the note of July 23, 1914, presented by the Austro-Hungarian Minister at Belgrade, the Imperial and Royal Government are themselves compelled to see to the safeguarding of their rights and interests, and, with this object, to have recourse to force of arms. Austria-Hungary consequently considers herself henceforward in a state of war with Serbia.’

Now then, let’s see about that somewhat elusive border skirmish.

Part 1 of 2