First To Mobilize?

Students of the Great War sometimes hear that in 1914 mobilizing a nation's armed forces was the equivalent to declaring war. The trains, it was said, couldn't be turned back once they were rolling. While this is an exaggeration of the 1914 conditions in the literal sense, the serial triggering of ultimatums together with the difficulty of reversing the process of moving troops to the front probably overwhelmed the various governments. So — who did mobilize first? Who tilted that first domino over? 

Officers and Men of the Serbian Army on Parade, Prewar

The answer is that it was Serbia on 25 July 1914, the same day it replied to the onerous Austro-Hungarian ultimatum. Most likely, they figured whatever answer they made was bound to be deemed as inadequate, so they ought to get ready for what was coming. Austria-Hungary responded with their own mobilization the next day, and the race to Armageddon was on. 

Serbian Army Moving to the Border Shortly after Mobilization