Which two countries declared war on each other at the start of WWI in 1914?

In 1914, the initial declaration of war that marked the beginning of World War I was between Austria-Hungary and Serbia. This conflict was ignited by the assassination of Archduke Franz Ferdinand of Austria on June 28, 1914, which led to Austria-Hungary issuing an ultimatum to Serbia. When Serbia's response was deemed unsatisfactory, Austria-Hungary declared war on July 28, 1914, officially starting the war.

The significance of this declaration lies in its role as a trigger for the larger conflict. The alliances and entanglements between various countries soon led to a domino effect, with other nations being drawn into the war. It's crucial to note that while other countries like Germany, Russia, and France became involved shortly afterward, the very first formal declaration of war that set everything into motion was indeed between Austria-Hungary and Serbia.