Final answer:
The assassination of Archduke Franz Ferdinand was the event that triggered World War I, leading to a series of declarations of war due to complex alliances, and setting the stage for a devastating global conflict.
Step-by-step explanation:
The Spark that Ignited World War I
The single event that set in motion the start of World War I was a) The assassination of Archduke Franz Ferdinand. On June 28, 1914, in an era brimming with nationalism and complex alliances, the heir to the Austro-Hungarian Empire and his wife were killed by Gavrilo Princip, a Serbian nationalist. This assassination did not simply claim the lives of the Archduke and his wife; it disrupted the precarious balance of alliances and mutual defense agreements that had existed in Europe.
Chain Reaction Leading to War
The murder of the Archduke Franz Ferdinand led Austria-Hungary to issue a series of demands to Serbia, which were not fully met. In response, Austria-Hungary declared war on Serbia. This set off a domino effect due to the alliance systems: Russia declared war on Austria-Hungary, Germany declared war on Russia and France, and Britain declared war on Germany. The entangled alliances pulled the major powers into conflict, and by the end of August 1914, what would come to be known as The Great War, or World War I, was underway.
The First Few Months of the War
The initial months of World War I were marked by rapid movements of troops and aggressive strategies. Germany’s plan to quickly defeat France before focusing on Russia, known as the Schlieffen Plan, failed to achieve a decisive victory. Instead, a stalemate ensued with trench warfare that would characterize much of the war. The war that many had believed would be over by Christmas of 1914 quickly turned into a drawn-out and devastating conflict affecting millions.