Was it the work of revolutionaries like Lenin and Trotsky? Certainly not - they were mostly either in prison or in exile. Lenin had said in 1916 that he feared he would not live to see a revolution in Russia!
Was it the War? The war certainly had a serious impact on all aspects of Russian society. Defeats undermined the army, and economic problems alienated much of the population.
Lack of food and fuel in the cruel winter of 1916-1917 certainly caused many strikes, unrest and disruption. Inflation meant people's savings were being eroded.
War production meant fewer consumer goods. Mobilisation limited food production. Transport difficulties made things worse. Certainly the war had a major part to play in the revolution of February 1917.
Or did the War merely hasten things that were already happening in Russia? Peasant unrest because of land shortages, worker unrest because of low wages, long hours, poor living conditions were long-standing issues.
The middle-class and some nobles wanted political reform, as we have already seen.
Revolutionaries had been active in Russia for many years, even assassinating the Tsar in 1881. Did the War help to make these pressures too much for Tsarism to bear?
Perhaps the War hindered revolution? In 1914 most of the country rallied behind Nicholas II in a great surge of patriotism.
Food production increased in 1914, 1915 and 1916. Perhaps without the war there might have been a revolution in Russia sooner?
Was it Nicholas II himself? What part did he play in his overthrow? He was a weak and indecisive leader - not what you want in an autocrat.
He found it hard to decide what was best to do. He relied heavily on his wife and family for support. He did the honourable thing and went to lead his armies in a horrible war.
He desperately wanted to retain the autocracy. He tried hard to change Russia, to make it stronger and more powerful, but was not