What was the main result of Austria-Hungary's invasion of Serbia?

The invasion of Serbia by Austria-Hungary is widely recognized as a key event that triggered the outbreak of World War I. This military action was part of a broader response to the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, by a Serbian nationalist in 1914. Austria-Hungary's declaration of war on Serbia set off a chain reaction of alliances and hostilities among the major European powers. The country found itself entangled in a complex network of alliances, leading to the mobilization of Russia in defense of Serbia, followed by declarations of war between several states. This escalation ultimately resulted in a full-scale global conflict, marking the beginning of the war that would involve many nations and result in significant geopolitical changes world over.

The other provided choices do not accurately encapsulate the primary outcome of the invasion. While the expansion of the Austro-Hungarian Empire and potential alliances with other powers were discussed, they were ultimately overshadowed by the widespread conflict that erupted, illustrating that the invasion's most significant consequence was igniting World War I.