Final answer:
World War I began with Austria-Hungary's invasion of Serbia following the assassination of Archduke Franz Ferdinand by a Serbian nationalist in Sarajevo. The event activated a network of alliances that led to several nations, including Germany, Russia, and Great Britain, entering the war.
Step-by-step explanation:
The military action that led to the start of World War I was Austria-Hungary's invasion of Serbia. This invasion was a direct result of the assassination of Archduke Franz Ferdinand. He was murdered in Sarajevo, Bosnia, by Gavrilo Princip, a Serbian nationalist, on June 28, 1914. The assassination caused political turmoil and led Austria-Hungary to present an ultimatum to Serbia, which, when not fully accepted, resulted in Austria-Hungary declaring war on Serbia on July 28, 1914. This conflict initiated the mobilization of aligned forces, with Russia mobilizing in defense of Serbia, which in turn led to Germany declaring war on Russia and France, and subsequently, the involvement of Great Britain after Germany's invasion of Belgium.
The chain reaction of alliances and treaties pulled the major European powers into the conflict, spiraling into a full-scale global war. The first few months of the war involved the movement of forces, declarations of war by various nations, and included key battles such as the Battle of the Frontiers and the First Battle of the Marne, which prevented the quick victory that Germany had hoped for. The hope for a swift resolution faded, setting the stage for a prolonged and grueling war.