Tsar Nikolai, or Nicholas, had faced domestic turmoil throughout his reign. Significantly, he faced a bloody revolution in 1905. But it was Russian involvement in the First World War that proved fatal to his rule, and disastrous to the Romanov dynasty as a whole.
The war went disastrously for the Russian army. Wartime sacrifices led to privations at home, especially in cities like Moscow and St. Petersburg. The Tsar's inability to manage the war led to discontent among court nobles and the people more generally. In February of 1917, mass protests in St. Petersburg erupted. Riots actually increased in size and intensity when the Tsar, in a repeat of the events of 1905, ordered the army to crush them.
Eventually the army itself stood down. Having lost all support, the Tsar abdicated in mid-March. This was a historical event, which marked the end of three centuries of Romanov rule. In his place rose a moderate liberal government under Alexander Kerensky. The new liberal government only lasted a few months in the face of the Bolshevik Revolution.
In 1918, the Tsar and his immediate family were executed by the Bolsheviks, ending any hope of a return to tsarist rule. In the end, the Tsar was undone by his own inability to deal with the crisis of the war. The war had unleashed larger structural problems. Challenges and shortages revealed the extent to which Imperial Russia lagged behind the rest of Europe. Political unrest after the war ended was the final nail in Tsar Nikolai's proverbial coffin.