The assassination of Archduke Franz Ferdinand in Sarejevo on 28 June 1914, led to a month of diplomatic maneuvering between Austria-Hungary, Germany, Russia, France and Britain, called the July Crisis. Austria-Hungary correctly believed that Serbian officials were involved in the assassination and on 23 July sent Serbia an ultimatum to Serbia intended to provoke a war. This led to Austria partially mobilizing, followed by Russia doing the same in support of Serbia. Austria declared war on Serbia on 28 July, and a series of partial mobilizations and diplomatic warnings followed, including Germany demanding Russia's demobilization, and warning France to remain neutral rather than come to Russia's aid. After various messages back and forth, misunderstandings, and erroneous assumptions about what other countries might do, Germany invaded Luxembourg and Belgium on 3–4 August, and Britain entered the war due to its 1839 treaty with Belgium. Europe was plunged into the Great War.