Which country was ruled by tsars until 1917?

Russia was ruled by tsars until the revolutions of 1917. The title derives from Caesar, adopted to claim the mantle of Rome by way of Byzantium after Constantinople fell, and it carried an explicitly religious as well as political authority. The last to hold it was Nicholas II, whose reign combined military defeat, an economy straining under the demands of the First World War, and a refusal to share power meaningfully with the Duma that had been conceded after earlier unrest. Bread shortages and strikes in the capital in February 1917 turned into a general revolt when troops refused to fire on crowds, and Nicholas abdicated. A provisional government took over but kept Russia in the war, and the Bolsheviks seized power in October, taking the country out of the fighting and into a civil war. The imperial family was killed in 1918.

Bulgaria and Serbia also used the title tsar at various points, borrowed from the same Byzantine tradition.