Although the system of alliances which underlay the geopolitics of late 19th and early 20th century Europe did not directly cause of the outbreak of war in 1914, it was critical in the formation of the political realities which did. Though nothing new, alliances between European states took on a new importance after the foundation Germany as a nation-state in 1871. The creation of Germany resulted in a monumental upset of power, and as such, Germany carefully allied herself with other Great Powers, most notable of which is the alliance which began in 1873 with Russia and Austria-Hungary: the Dreikaiserbund (Three Emperor’s League). This alliance disbanded and reformed more than once, but Austria-Hungary, Russia, and Germany were de facto allies until 1890. In 1890, the Reinsurance Treaty which linked Russia and Germany expired, and Kaiser Wilhelm II chose not to renew, believing his personal connection with the Russian Tsar would be enough to secure peace. This caused Russia to seek political ties with France in the Franco-Russian Alliance of 1892. When Britain unofficially allied with France in the Entente-Cordiale, this left Germany without major allies to both her eastern and western borders. This led Germany to adopt an aggressive defense policy known as Einkreisungspolitik, which in itself was major factor in the start of the war.
In the shorter term, the remaining alliance between Austria-Hungary and Germany was a much more pressing cause of the First World War. After the Austrian invasion of Serbia, and a Russian issued ultimatum, Germany provided Austria-Hungary with a carte-blanche to act as she pleased in Serbia with German support. This caused aggressive mobilization policies from Russia and led Germany to engage in a pre-emptive strike against Russia’s ally: France. This day, 3 August 1914, was the official start of the First World War. Furthermore, though not directly relevant to its outbreak, Britain’s justification for joining the war on August 4th was the German invasion of Belgium. Belgium had an alliance with Britain in the form of the 1839 Treaty of London, which German foreign minister Bethmann Hollweg deigned unimportant, describing it as a “scrap of paper”. The alliance system was therefore critical not only in the setting of the necessary political landscape for war, but also its immediate cause.