Political tensions over the rise of the German Empire were the war's principal cause, although it was set off by the assassination of Archduke Franz Ferdinand of Austria by a Bosnian Serb nationalist in Sarajevo, an event used as a pretext by Austria for declaring war on Serbia. Most of the fighting took place on land in Europe and was generally characterized by long periods of bloody stalemate; the balance eventually shifted in the Allies' favour in 1917 when the US joined the war. Total casualties of the war are estimated at 10 million killed. One of the consequences of the war was the collapse of the German, Austro-Hungarian, Russian, and Ottoman empires.