The Decembrist Revolt took place in Russia on 26 December [O.S. 14 December] 1825, following the sudden death of Emperor Alexander I. Alexander's heir-presumptive, Konstantin, had privately declined the succession, unknown to the court, and his younger brother Nicholas ascended the throne as Emperor Nicholas I. While some of the army had sworn loyalty to Nicholas, a force of about 3,000 troops tried to mount a military coup in favour of Konstantin. Wikipedia