Who declared war on Russia?

In WWI, the Alliance system was so confusing that a small dispute, by international standards at least, in Serbia caused a major conflict. In response to the assassination of the heir to the Austro-Hungarian throne, the Austro-Hungarian empire declared war on Serbia on 28th July.

Russia mobilised in support of the Serbs. In support of Austro-Hungary Germany declared war on Russia as the French mobilised to support Russians. 3 August: Germany declares war on France and then 4 August: Britain declares war on Germany.