Answer: Russia supported its ally Serbia.
Explanation:
After Austria-Hungary asked the commission to enter Serbia in June to investigate the murder of Franz Ferdinand, Serbia rejected the request. Austria-Hungary immediately declared war on Serbia. Austro-Hungary wanted to maintain its importance in the Balkans, and Serbia was a nuisance. After that act of Austro-Hungary, Russia immediately declared war on Austria-Hungary, considering Serbia was the main Russian ally in the Balkans.
