(A) Their heir was assassinated
(B) ** Their ally Serbia became involved in the conflict
(C) Germany invaded Russia
(D) Japan defeated Russia to start a war
EXPLANATIONS BELOW
Concept note-1: -When Austria invaded Serbia on July 28, 1914, Russian Foreign Minister Sergei Sazonov viewed it as an Austro-German conspiracy to end Russian influence in the Balkans. On July 30, Russia declared a general mobilization in support of Serbia.
Concept note-2: -Russia reluctantly entered the Great War to preserve its status as a great power. However, once the fighting began, it was the first amongst the Allies to state its territorial desiderata, which were to annex lands along the borders of Germany and Austria-Hungary.
Concept note-3: -World War I began with Austria-Hungary declaring war on Serbia. The Habsburg Empire had desperately been looking for a reason to do so for years, feeling that the Serbs were a little too active in the business of Bosnia and Herzegovina, the Germanic empire’s newest provinces.
Concept note-4: -The Russo-Serbian Alliance (Serbian: - / Rusko-srpski savez, Russian: - ) was signed on 10 July 1807 between Revolutionary Serbia under Đorđe Petrović (Karađorđe) and the Russian Empire, during the First Serbian Uprising.