Pugachev's Rebellion (or the Cossack Rebellion) of 1773-75 was the principal revolt in a series of popular rebellions that took place in Russia after Catherine II seized power in 1762. It began as an organized insurrection of Yaik Cossacks headed by Yemelyan Pugachev, a disaffected ex-lieutenant of the Russian Imperial army, against a background of profound peasant unrest and war with the Ottoman Empire. After the initial success, Pugachev assumed leadership of an alternative government in the name of the assassinated Tsar Peter III and proclaimed an end to serfdom. This organized leadership presented a challenge to the imperial administration of Catherine II.