Conflict between Austria-Hungary and Serbia

Austrian emperor did not want to go to war when his nephew (the archduke) was killed, but his government in Vienna saw it as chance to crash Serbia for good. To avoid war Austrians sent Serbs an ultimatum (final set of demand), but the Serbs didn’t agree all of the terms, and war was declared on July 28, 1914. In Germany, Kaiser William II was horrified by the assassination, and gave Austria a “Blank check” to show its support.
Serbia meanwhile sought help from Russia, Nicholas II telegraphed William II to soften its demands, but his plea failed, he began to mobilize (preparing its troops for war). As response to this Germany declared war on Russia. Russia then turned to France, and though French had doubts they gave their support to Russia. Germany asked France to keep out of it and when they refused, Germany declared war on France.
Italy and France remained neutral. Neutrality is a policy of supporting neither side in a war. However after Germany invaded Belgium to succeed in their Schlieffen Plan, Britain declared war on Germany

Source: World History Text Book.