World War I
Austria-Hungary blamed Serbia for the assassination. Germany supported Austria-Hungary and promised full support in case of war.  Austria-Hungary sent the very harsh July Ultimatum to Serbia and threatened war unless Serbia agreed to all of its terms. Many historians think that Austria-Hungary already wanted a war with Serbia. Serbia agreed to most of the conditions, but Austria-Hungary still declared war on Serbia, which quickly led to a full-scale war. The countries had allies that entered the war in a matter of days.