Please answer a question about the following article about Modern history:

Under Peter I (the Great), Russia was proclaimed an Empire in 1721 and became recognized as a world power. Ruling from 1682 to 1725, Peter defeated Sweden in the Great Northern War, forcing it to cede West Karelia and Ingria (two regions lost by Russia in the Time of Troubles), as well as Estland and Livland, securing Russia's access to the sea and sea trade. On the Baltic Sea Peter founded a new capital called Saint Petersburg, later known as Russia's Window to Europe. Peter the Great's reforms brought considerable Western European cultural influences to Russia. Catherine II (the Great), who ruled in 1762–96, extended Russian political control over the Polish-Lithuanian Commonwealth and incorporated most of its territories into Russia during the Partitions of Poland, pushing the Russian frontier westward into Central Europe. In the south, after successful Russo-Turkish Wars against the Ottoman Empire, Catherine advanced Russia's boundary to the Black Sea, defeating the Crimean khanate.

Under whom was Russia declared an empire?
Peter I