The answer is A: Nations mobilize their military forces in response to threats to their allies
The mobilization took place really fast, and it developed this way:
<u>28 June:</u> After Gavrilo Principe assasinated Archduke Franz Ferdinand, Austrian people were determined to attack Serbia.
<u>5 July:</u> Kaiser Wilhem (William) II, guaranteed German people would support to Austria in its measures against Serbia, via what it was called "Blank Cheque".
<u>23 July:</u> Austrians gave an ultimatum to Serbians, and they had to reply within 48 hours. Even though the text's approval took place on the July 19, its presentation was delayed until French President and Prime Minister left Russia after their visit. This was a preventive move so as to avoid a coordinated response from the Russians and the French. The response was effectively made when the French delegates were at sea.
<u>28 July:</u> The Serbian reply was rejected and Austria declared war. The Russians partially mobilised their troops against Austria, defending Serbia.
<u>29 July:</u> Austria bombarded Belgrade.
<u>30 July</u>: Russia commanded general mobilisation and British efforts to mediate failed.
<u>31 July</u>: Russia was presented an ultimatum by Germany to stop its mobilisation within 12 hours. Germany also presented one to France in order to ask for neutrality and that they should give over borders fortresses as guarantee.
German militar forces thought that once war unleashed between them and Russia, war with France was inevitable. A plan called the <em>Schlieffen plan</em> was executed by Germans, and it consisted of a concentration of German forces to attack France.
<u>1 August:</u> The Germans declared war on Russia. France mobilised its troops.
<u>2 August</u>: Germany demanded Belgium the right to mobilise troops by crossing their country. They refused.
<u>3 August</u>: Germany declared war on France and its troops crossed Belgium. Britain demanded the evacuatiom of Belgium via an ultimatum to Germany.
<u>4 August</u>: The British declared war on Germany.
