1914
Franz Ferdinand
In World War 1 France declared war on the central powers which included Austria-Hungary two countries combined under one monarch.
Austria-Hungary
Serbia
6th august 1914
Assassination of Archduke Franz Ferdinand of Austria .
in 1914
Because Russia Invaded Austria.
Check out http://www.firstworldwar.com/features/declarationsofwar.htm Here's how the dominos fell in WWI: Archduke Franz Ferdinand of Austria assassination on 28 June 1914 Austria-Hungary declared war on Serbia on 28 July 1914 Russia, Germany & France mobilize over the next few days. Germany declared war on Russia on 1 Aug 1914 Canada, France, Russia, Belgium, and the UK were all in a state of war with Germany, A-H within 2 weeks. Italy, Greece, Turkey and the USA entered the war somewhat later.
28 July 1914 - Austria-Hungary declares war on Serbia 6 August 1914 - Austria-Hungary declares war on Russia 23 Maj 1915 - Italy declares war on Austria-Hungary
Belgium. Neutral Belgium was attacked by the Germans on there way to France