Fighting commenced when Austria invaded Serbia on 28 July 1914, purportedly in response to the assassination of Archduke Franz Ferdinand, heir to Emperor Franz Joseph; this brought Serbia's ally Montenegro into the war on 8 August and it attacked the Austrian naval base at Cattaro, modern Kotor. At the same time, German troops carried out Schlieffen Plan entering neutral Belgium and Luxembourg; over 95% of Belgium was occupied but the Belgian Army held their lines on the Yser Front throughout the war. This allowed Belgium to be treated as an Ally, in contrast to Luxembourg which retained control over domestic affairs but was occupied by the German military.