Illarionov

"The crucial issue that has continued to provoke controversy among
historians of Eastern Slavdom over the last two hundred years is whether
the Kyivan rulers and elites managed to complete their project before the
destruction of the Rus state. Did they succeed in shaping a coherent Rus nationality that later gave birth to the three modern nations of Russia,
Ukraine, and Belarus? Or was the East Slavic world divided from the very
beginning, and did the three nations already exist in Kyivan times? The
evidence presented in this book supports a negative answer to both questions."

Serhii Plokhy. The Origins of the Slavic Nations: Premodern Identities in Russia, Ukraine, and Belarus. — Cambridge: Cambridge Universiy Press, 2006. — 379 pp.
Источник - из википедии, статья "Древнерусская народность".