Alliances were an important cause of World War I because they transformed what was a regional crisis in the Balkans into a broader European war. This was because the major powers were enmeshed in a complex system of alliances that obliged them to enter the conflict. When a Bosnian Serb group known as the Black Hand assassinated Archduke Franz Ferdinand of Austria in the summer of 1914, Austria-Hungary, with the support of Germany, held the nation of Serbia itself accountable. They issued an ultimatum to the Serbs that that nation could not accept, and when they declined, they declared war. The problem was that Serbia had an alliance with Russia, who mobilized its troops against Austria-Hungary in response. This led Germany to honor its commitment to the Austrians by declaring war against Russia.
Russia, in the meantime, had an alliance with France that caused the French to enter the war. Germany, faced with the challenge of fighting a two-front war, invaded France, attacking Belgium in the process. The attack on Belgium triggered a treaty between that nation and Great Britain, who declared war on the Germans. So alliance systems set in motion a chain of events that culminated with a continent-wide war beginning in the summer of 1914. Other nations, including the Ottoman Empire and later the United States, would join the fray later.