The assassination of Archduke Franz Ferdinand, heir to the throne of Austria-Hungary, on June 28, 1914, was the event that directly led to Britain's entry into World War I.
The Assassination
Archduke Franz Ferdinand and his wife, Sophie, Duchess of Hohenberg, were visiting Sarajevo, the capital of Bosnia-Herzegovina (which had been annexed by Austria-Hungary in 1908), when they were attacked by a group of Bosnian Serbs who were members of the Black Hand, a secret society dedicated to the unification of all South Slavic peoples into a single state.
One of the assassins, Gavrilo Princip, shot and killed both Franz Ferdinand and Sophie as they were traveling in a motorcade through the city. The assassination sparked outrage and indignation throughout Europe, and it led to a series of diplomatic crises between Austria-Hungary and Serbia.
Austria-Hungary's Ultimatum to Serbia
In response to the assassination, Austria-Hungary issued an ultimatum to Serbia on July 23, 1914, demanding that it accept a series of conditions, including the punishment of those responsible for the assassination and the suppression of anti-Austrian propaganda in Serbia.
Serbia agreed to most of the conditions, but it refused to accept some of the more humiliating demands, such as allowing Austrian officials to investigate the assassination on Serbian soil. As a result, Austria-Hungary declared war on Serbia on July 28, 1914.
Russia's Mobilization and Germany's Declaration of War
Russia, which was allied with Serbia, responded to Austria-Hungary's declaration of war by mobilizing its military. This alarmed Germany, which had a treaty with Austria-Hungary that required it to come to its aid if it was attacked.
Germany issued an ultimatum to Russia demanding that it halt its mobilization, but Russia refused. As a result, Germany declared war on Russia on August 1, 1914.
France's Declaration of War and Britain's Entry into the War
France, which was allied with Russia, declared war on Germany on August 3, 1914. Britain, which was allied with France and Belgium (which had been invaded by Germany), declared war on Germany on August 4, 1914.
Thus, the assassination of Archduke Franz Ferdinand was the event that directly led to Britain's entry into World War I.