Original sentence: By the beginning of the Age of Discovery, many of the former principalities of Kievan Rus were integrated by the Grand Duchy of Moscow.Following the settlement of East Slavs in the East European Plain in the middle of the 1st millennium BC, through the next thousand years, most of European Russia came into the sphere of Slavic cultural and political influence, and finally became a part of the Russian state.Modern era polar icebreakers, dating from Stepan Makarov's Yermak, made Arctic voyages safer and led to new attempts to explore the Northern Sea Route.The last major unknown archipelago on Earth, Severnaya Zemlya, was discovered by Boris Vilkitsky during his 1913 expedition on the icebreakers Taymyr and Vaygach and later explored and mapped in 1931 by Nikolay Urvantsev and Georgy Ushakov
Edited sentence: By the beginning of the Age of Discovery, most of the former principalities of Kievan Rus were re-integrated by the Grand Duchy of Moscow.Following the settlement of East Slavs in the Russian Plain in the middle of the 1st millennium BC, through the next thousand years, most of European Russia came into the sphere of Slavic cultural and political influence, and finally became a part of the Russian state.Modern era polar icebreakers, dating from Stepan Makarov's Yermak, made  Arctic voyages safer and led to new attempts to explore the Northern Sea Route.The last major unknown archipelago on Earth, Severnaya Zemlya, was discovered by Boris Vilkitsky during his expedition on the icebreakers Taymyr and Vaygach and later explored and mapped by Nikolay Urvantsev and Georgy Ushakov