Russia entered World War One shortly after it started. Austria- Hungary had given Serbia an ultimatum the small country couldn't possibly accomplish. When Serbia didn't comply with the demands, Austria-Hungary declared war on Serbia. At the time, Russia believed in a principle called Pan-Slavism. This was a form of nationalism in which they believed all Slavic peoples shared a common bond through their nationality. Russia felt a need to defend the smaller country and, in turn, declared war on Austria-Hungary.