Austria delivered an ultimatum to Serbia on 23rd July. Its harsh terms were intended to be unacceptable, for, in effect, they demanded that Austria be given control over Serbian internal affairs.
Next day, Germany caused consternation in Russia, Great Britain, and France when it delivered notes to each, stating that the Austrian demands were fair and which also warned these Countries not to interfere in Austro-Serbian affairs.
Unexpectedly, Serbia acceded to most of the Austrian demands, and, on hearing this, the Kaiser, thinking that threats had been enough, was satisfied.
Too late! Momentum now controlled reason and the destinies of young men moved inexorably from the light into the shadow of death. A range of threats, proposals, understandings, notes, and reports all jumbled together and produced no solution.
The Explosion
At 11 a.m. on 28th July Austria pre-empted further negotiation and declared war on Serbia.
The generals of all the rival Continental nations had their war plans, and military necessity and the mobilisation of armies now took precedence over the last hopes of politicians. On 31st both the Emperors of Russia and Austria ordered a mobilisation of all their respective armies. Germany started mobilising on 1st August.
http://users.tibus.com/the-great-war/road_to.htm