The best answer is the garrison in Petrograd joining the Soviets. On 12 March 1917, revolutionaries set up the Petrograd Soviet with 2500 elected deputies. They began taking control of the food supplies in the city. They also set up soldier's committees, undermining the authority of the officers. WWI caused huge discontent in the countryside, which ultimately led Tsar Nicholas II to abdicate on March 15, 1917.
