February Revolution: After several days of rioting by hungry protestors in Petrograd, Tsar Nicholas II orders the army to use force to put an end to the problems. Out of the nearly 200,000 troops in and around the capitol, but only a little more than 10,000 were likely healthy, mobile, trained, and reliable — and even these, when called out, proved reluctant to fire on unarmed, hungry protestors.
Officers who couldn't or wouldn't lead troops against the protestors were summarily executed, effectively eliminating any organized leadership and thus eliminating even a theoretical ability of the army to maintain order. Tsar Nicholas II discontinued the Duma, eliminating their ability to govern or give legal orders and thus having a similar effect.
Very quickly, all government authority is eliminated in the Russian capitol, replaced with hastily organized Soviet councils of workers and defecting soldiers. When Tsar Nicholas II finally manages to return to Petrograd in a few days, he will be convinced to abdicate.