What was the second nation to declare war in World War 1?

The second nation to declare war in World War I was Austria-Hungary. Following the assassination of Archduke Franz Ferdinand on June 28, 1914, Austria-Hungary issued an ultimatum to Serbia. When Serbia's response was deemed unsatisfactory, Austria-Hungary declared war on Serbia on July 28, 1914, prompting a series of alliances and declarations of war that expanded the conflict.