Final answer:
Russian expansion into the Balkans threatened the regional power balance, particularly challenging Austro-Hungarian and Ottoman influence, and contributed to the tensions leading up to World War I.
Step-by-step explanation:
The Russians' desire to spread into the Balkans threatened the power dynamics and territorial control in the region, particularly posing a challenge to the Austro-Hungarian and Ottoman Empires. This expansionist motive aligned with the concept of pan-Slavic nationalism, where Russia saw itself as the protector and unifier of Slavic peoples, also yearned for a warm-water port with direct access to the Mediterranean Sea. The situation intensified when Serbia, emboldened by Russian support and its own nationalist ambitions, sought to gain territory in the Balkans, which led to a conflict of interest with Austria-Hungary, especially over Bosnia, where many Serbs resided. These tensions among nationalistic drives, imperial declines, and alliance formations significantly contributed to the onset of the First World War.