Final answer:
Czar Nicholas II abdicated due to military defeats and domestic issues, paving the way for the Bolsheviks under Vladimir Lenin to seize power. The subsequent Russian Civil War led to the Bolshevik victory and the establishment of the Soviet Union, which underwent centralization and collectivization under Stalin.
Step-by-step explanation:
Czar Nicholas II lost power amid the turmoil of World War I and the Russian Revolution of 1917. Facing military defeats and domestic strife, Nicholas abdicated the throne in March 1917. The provisional government that followed proved ineffectual, paving the way for the Bolsheviks, led by Vladimir Lenin, to seize power in the October Revolution. By 1918, the former Czar and his family were executed, eliminating the possibility of a monarchist restoration.
The Russian Civil War broke out in 1918 following the Bolshevik takeover, with the Red Army, supporting the Bolshevik government, fighting against the White Army, which opposed Lenin's socialist vision. This lengthy conflict, which resulted in the deaths of nearly ten million people, ended with a victory for the Bolsheviks and solidified their control over Russia.