It sounds like you're referencing the events that led to the outbreak of World War I in 1914.
On June 28, 1914, Archduke Franz Ferdinand of Austria was assassinated in Sarajevo. This event set off a chain reaction of political tensions and alliances that culminated in the declaration of war. Austria-Hungary issued an ultimatum to Serbia, which led to Austria-Hungary declaring war on Serbia.
In response, Russia mobilized in support of Serbia. Germany, allied with Austria-Hungary, declared war on Russia on August 1, 1914, and then declared war on France on August 3, 1914. This series of events quickly escalated into a global conflict, involving many countries and leading to years of devastating warfare.
If you have any specific questions about these events or their consequences, feel free to ask!