Final answer:
Russia left World War I due to massive casualties and internal strife, leading to the rise of the Bolsheviks under Vladimir Lenin, who established a communist government. The Treaty of Brest-Litovsk ended Russian involvement in the war.
Step-by-step explanation:
Russia withdrew from World War I due to enormous casualties, military failures, and domestic issues, including the Tsar's abdication following the February Revolution. Vladimir Lenin and the Bolsheviks came to power in the subsequent October Revolution, resulting in a far-reaching government change to communism, forming the USSR.
The Russian withdrawal had significant implications for the structure of Europe. Following the revolution and civil war, Russia transformed from the Tsarist autocracy into the world's first communist state, promising to establish a 'dictatorship of the proletariat'. This ideological shift significantly altered the geopolitical landscape of the post-war era.