The Bolshevik Revolution of 1917 in Russia was

caused in part by

(1) a forced famine in Ukraine

(2) the failure of Czar Nicholas II to come to

Serbia's aid

(3) a shortage of military supplies and food during

World War I

(4) the establishment of Lenin's New Economic

Policy (NEP)

I would go for (3) A shortage of military supplies and food ...

This also because the first sparks of rebellion came from the army after the terrible sufferings of the soldiers (killed by the hundred thousands because of poor directions from the officers and almost no supply in the field). It is interesting to remember that the famous "Cruiser Potemkin" mutiny was started (allegedly) by the distribution to the sailors of rotten meat!