When Austria-Hungary declared war on Serbia, on July 28, 1914, they inadvertently declared war across Europe, as nations became trapped by a tangled web of alliances.
Germany – bound by the Triple Alliance pact between Russia, Germany and Italy – had offered Austria a “blank cheque” guarantee should Serbia seek revenge for the assassination of Franz Ferdinand.
Britain’s Foreign Secretary, Sir Edward Grey, pleaded that Germany, France, Italy and Britain should “act together for the sake of peace simultaneously” – but he was ignored.
On July 29, Austria bombed the Serbian capital Belgrade. Russia mobilised her troops. Germany ordered Russia to stop, or they’d be at war.
This dragged France and Britain further into the growing crisis; both countries had signed the Entente Cordiale in 1907 with Russia.
Britain was bound by another pact too; the 1839 Treaty of London, when Britain had promised to defend Belgium’s neutrality.
Panicked, Britain asked France and Germany to honour Belgium’s wishes not to be drawn into war. France agreed, but Germany didn’t respond and instead, on August 1, they declared war on Russia, and two days later, having positioned troops on French borders, Germany declared war on France too.
On August 3, Germany warned that unless Belgium allowed German troops across her lands to reach France, they would “treat her as an enemy”. Belgium refused but Germany simply marched straight through.
The next morning, Britain issued an ultimatum to Germany, ordering them to leave Belgium. They didn’t, and on August 4 - 11pm British time, midnight German time - the two countries commenced hostilities.
The First World War had officially begun.