Catherine the Great was German by birth; she came to Russia to be the arranged bride of her cousin Peter who had been selected by childless Empress Elizabeth of Russia to be her successor. Catherine converted to the Russian Orthodox Church and she and Peter were married, but it was not a fulfilling union and the children she eventually bore were almost certainly not fathered by Peter.
When Peter proved to be an inept ruler, Catherine used the support of her Russian lover's family, the Church, and the military to lead a bloodless coup that forced Peter to abdicate the throne to Catherine. She justified the move as being what the people wished and as allowing her to reestablish the authority of the Russian Orthodox Church.
As Empress, Catherine worked hard to open up Russia to new relationships with western European countries, which she recognized were innovating and developing new practices in trade and government that would benefit her country. She rewrote much of the legal and social structure of Russia and gave new standing to the upper classes of Russia's population, but ignored the plight of the majority of the people who continued to live as serfs. She had to deal with frequent wars but added to Russia's territory as a result of most conflicts.